<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:c52ba12a-d86b-4e35-8336-06172e7eabf1,g:826f2442-19ab-4ea6-b645-c89e3571f702-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:iclr="http://www.iconplc.com/20220331" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2020-01-21" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.iconplc.com/20220331">
  <xs:import namespace="http://fasb.org/srt/2021-01-31" schemaLocation="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2021-01-31" schemaLocation="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd"/>
  <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="http://www.xbrl.org/2003/xlink-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2020-01-21" schemaLocation="https://www.xbrl.org/dtr/type/2020-01-21/types.xsd"/>
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/country/2021" schemaLocation="https://xbrl.sec.gov/country/2021/country-2021.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2021" schemaLocation="https://xbrl.sec.gov/dei/2021/dei-2021.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="iclr-20220331_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="iclr-20220331_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="iclr-20220331_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="iclr-20220331_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="Cover" roleURI="http://www.iconplc.com/role/Cover">
        <link:definition>0001001 - Document - Cover</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDBALANCESHEETS" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS">
        <link:definition>1001002 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical">
        <link:definition>1002003 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSUNAUDITED">
        <link:definition>1003004 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUNAUDITED" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUNAUDITED">
        <link:definition>1004005 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED">
        <link:definition>1005006 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED" roleURI="http://www.iconplc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED">
        <link:definition>1006007 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Basisofpresentation" roleURI="http://www.iconplc.com/role/Basisofpresentation">
        <link:definition>2101101 - Disclosure - Basis of presentation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BasisofpresentationDetails" roleURI="http://www.iconplc.com/role/BasisofpresentationDetails">
        <link:definition>2402401 - Disclosure - Basis of presentation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Significantaccountingpolicies" roleURI="http://www.iconplc.com/role/Significantaccountingpolicies">
        <link:definition>2103102 - Disclosure - Significant accounting policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SignificantaccountingpoliciesPolicies" roleURI="http://www.iconplc.com/role/SignificantaccountingpoliciesPolicies">
        <link:definition>2204201 - Disclosure - Significant accounting policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Revenue" roleURI="http://www.iconplc.com/role/Revenue">
        <link:definition>2105103 - Disclosure - Revenue</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueTables" roleURI="http://www.iconplc.com/role/RevenueTables">
        <link:definition>2306301 - Disclosure - Revenue (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RevenueDetails" roleURI="http://www.iconplc.com/role/RevenueDetails">
        <link:definition>2407402 - Disclosure - Revenue (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities" roleURI="http://www.iconplc.com/role/Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities">
        <link:definition>2108104 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables">
        <link:definition>2309302 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofAccountsReceivablesandUnbilledRevenueDetails" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofAccountsReceivablesandUnbilledRevenueDetails">
        <link:definition>2410403 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Accounts Receivables and Unbilled Revenue (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofUnbilledServicesandUnearnedRevenueorPaymentsonAccountDetails" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofUnbilledServicesandUnearnedRevenueorPaymentsonAccountDetails">
        <link:definition>2411404 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Unbilled Services and Unearned Revenue or Payments on Account (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails">
        <link:definition>2412405 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesNarrativeDetails_1">
        <link:definition>2412405 - Disclosure - Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Goodwill" roleURI="http://www.iconplc.com/role/Goodwill">
        <link:definition>2113105 - Disclosure - Goodwill</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="GoodwillTables" roleURI="http://www.iconplc.com/role/GoodwillTables">
        <link:definition>2314303 - Disclosure - Goodwill (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="GoodwillScheduleofGoodwillDetails" roleURI="http://www.iconplc.com/role/GoodwillScheduleofGoodwillDetails">
        <link:definition>2415406 - Disclosure - Goodwill - Schedule of Goodwill (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="GoodwillNarrativeDetails" roleURI="http://www.iconplc.com/role/GoodwillNarrativeDetails">
        <link:definition>2416407 - Disclosure - Goodwill - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Intangibleassets" roleURI="http://www.iconplc.com/role/Intangibleassets">
        <link:definition>2117106 - Disclosure - Intangible assets</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleassetsTables" roleURI="http://www.iconplc.com/role/IntangibleassetsTables">
        <link:definition>2318304 - Disclosure - Intangible assets (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IntangibleassetsDetails" roleURI="http://www.iconplc.com/role/IntangibleassetsDetails">
        <link:definition>2419408 - Disclosure - Intangible assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Businesscombinations" roleURI="http://www.iconplc.com/role/Businesscombinations">
        <link:definition>2120107 - Disclosure - Business combinations</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsTables" roleURI="http://www.iconplc.com/role/BusinesscombinationsTables">
        <link:definition>2321305 - Disclosure - Business combinations (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsNarrativeDetails" roleURI="http://www.iconplc.com/role/BusinesscombinationsNarrativeDetails">
        <link:definition>2422409 - Disclosure - Business combinations - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails" roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofPRAHealthSciencesIncAcquisitionConsiderationDetails">
        <link:definition>2423410 - Disclosure - Business combinations - Schedule of Fair Value of PRA Health Sciences Inc Acquisition Consideration (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails" roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofPRAHealthSciencesIncAcquisitionDetails">
        <link:definition>2424411 - Disclosure - Business combinations - Schedule of PRA Health Sciences Inc Acquisition (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails" roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofFairValueofIntangibleAssetsPRAHealthSciencesIncAcquisitionDetails">
        <link:definition>2425412 - Disclosure - Business combinations - Schedule of Fair Value of Intangible Assets PRA Health Sciences Inc Acquisition (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails" roleURI="http://www.iconplc.com/role/BusinesscombinationsScheduleofProformaInformationofPRAHealthSciencesIncAcquisitionDetails">
        <link:definition>2426413 - Disclosure - Business combinations - Schedule of Proforma Information of PRA Health Sciences Inc Acquisition (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Equitymethodinvestments" roleURI="http://www.iconplc.com/role/Equitymethodinvestments">
        <link:definition>2127108 - Disclosure - Equity method investments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquitymethodinvestmentsTables" roleURI="http://www.iconplc.com/role/EquitymethodinvestmentsTables">
        <link:definition>2328306 - Disclosure - Equity method investments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquitymethodinvestmentsNarrativeDetails" roleURI="http://www.iconplc.com/role/EquitymethodinvestmentsNarrativeDetails">
        <link:definition>2429414 - Disclosure - Equity method investments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquitymethodinvestmentsSummaryofEquityMethodInvestmentsDetails" roleURI="http://www.iconplc.com/role/EquitymethodinvestmentsSummaryofEquityMethodInvestmentsDetails">
        <link:definition>2430415 - Disclosure - Equity method investments - Summary of Equity Method Investments (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Fairvaluemeasurements" roleURI="http://www.iconplc.com/role/Fairvaluemeasurements">
        <link:definition>2131109 - Disclosure - Fair value measurements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairvaluemeasurementsDetails" roleURI="http://www.iconplc.com/role/FairvaluemeasurementsDetails">
        <link:definition>2432416 - Disclosure - Fair value measurements (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Restructuring" roleURI="http://www.iconplc.com/role/Restructuring">
        <link:definition>2133110 - Disclosure - Restructuring</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringTables" roleURI="http://www.iconplc.com/role/RestructuringTables">
        <link:definition>2334307 - Disclosure - Restructuring (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringNarrativeDetails" roleURI="http://www.iconplc.com/role/RestructuringNarrativeDetails">
        <link:definition>2435417 - Disclosure - Restructuring - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringSummaryofRestructuringChargesDetails" roleURI="http://www.iconplc.com/role/RestructuringSummaryofRestructuringChargesDetails">
        <link:definition>2436418 - Disclosure - Restructuring - Summary of Restructuring Charges (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringSummaryofDetailsofMovementinRestructuringChargesDetails" roleURI="http://www.iconplc.com/role/RestructuringSummaryofDetailsofMovementinRestructuringChargesDetails">
        <link:definition>2437419 - Disclosure - Restructuring - Summary of Details of Movement in Restructuring Charges (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Operatingleases" roleURI="http://www.iconplc.com/role/Operatingleases">
        <link:definition>2138111 - Disclosure - Operating leases</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OperatingleasesTables" roleURI="http://www.iconplc.com/role/OperatingleasesTables">
        <link:definition>2339308 - Disclosure - Operating leases (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OperatingleasesScheduleofLeaseCostsDetails" roleURI="http://www.iconplc.com/role/OperatingleasesScheduleofLeaseCostsDetails">
        <link:definition>2440420 - Disclosure - Operating leases - Schedule of Lease Costs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OperatingleasesNarrativeDetails" roleURI="http://www.iconplc.com/role/OperatingleasesNarrativeDetails">
        <link:definition>2441421 - Disclosure - Operating leases - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OperatingleasesScheduleofOperatingLeaseMaturityDetails" roleURI="http://www.iconplc.com/role/OperatingleasesScheduleofOperatingLeaseMaturityDetails">
        <link:definition>2442422 - Disclosure - Operating leases - Schedule of Operating Lease Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OperatingleasesScheduleofOperatingLeaseMaturityDetails_1" roleURI="http://www.iconplc.com/role/OperatingleasesScheduleofOperatingLeaseMaturityDetails_1">
        <link:definition>2442422 - Disclosure - Operating leases - Schedule of Operating Lease Maturity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Bankcreditlinesandloanfacilities" roleURI="http://www.iconplc.com/role/Bankcreditlinesandloanfacilities">
        <link:definition>2143112 - Disclosure - Bank credit lines and loan facilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BankcreditlinesandloanfacilitiesTables" roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesTables">
        <link:definition>2344309 - Disclosure - Bank credit lines and loan facilities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesScheduleofOutstandingDebtDetails">
        <link:definition>2445423 - Disclosure - Bank credit lines and loan facilities - Schedule of Outstanding Debt (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BankcreditlinesandloanfacilitiesNarrativeDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesNarrativeDetails">
        <link:definition>2446424 - Disclosure - Bank credit lines and loan facilities - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BankcreditlinesandloanfacilitiesScheduleofContractualMaturitiesofDebtDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesandloanfacilitiesScheduleofContractualMaturitiesofDebtDetails">
        <link:definition>2447425 - Disclosure - Bank credit lines and loan facilities - Schedule of Contractual Maturities of Debt (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Incometaxes" roleURI="http://www.iconplc.com/role/Incometaxes">
        <link:definition>2148113 - Disclosure - Income taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncometaxesTables" roleURI="http://www.iconplc.com/role/IncometaxesTables">
        <link:definition>2349310 - Disclosure - Income taxes (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncometaxesScheduleofComponentsofIncomeTaxExpenseDetails" roleURI="http://www.iconplc.com/role/IncometaxesScheduleofComponentsofIncomeTaxExpenseDetails">
        <link:definition>2450426 - Disclosure - Income taxes - Schedule of Components of Income Tax Expense (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncometaxesNarrativeDetails" roleURI="http://www.iconplc.com/role/IncometaxesNarrativeDetails">
        <link:definition>2451427 - Disclosure - Income taxes - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Netincomeperordinaryshare" roleURI="http://www.iconplc.com/role/Netincomeperordinaryshare">
        <link:definition>2152114 - Disclosure - Net income per ordinary share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetincomeperordinaryshareTables" roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareTables">
        <link:definition>2353311 - Disclosure - Net income per ordinary share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetincomeperordinaryshareScheduleofReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareScheduleofReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails">
        <link:definition>2454428 - Disclosure - Net income per ordinary share - Schedule of Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetincomeperordinaryshareScheduleofReconciliationofNetIncomeAttributabletotheGroupDetails" roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareScheduleofReconciliationofNetIncomeAttributabletotheGroupDetails">
        <link:definition>2455429 - Disclosure - Net income per ordinary share - Schedule of Reconciliation of Net Income Attributable to the Group (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Sharebasedawards" roleURI="http://www.iconplc.com/role/Sharebasedawards">
        <link:definition>2156115 - Disclosure - Share-based awards</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsTables" roleURI="http://www.iconplc.com/role/SharebasedawardsTables">
        <link:definition>2357312 - Disclosure - Share-based awards (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsSummaryofStockOptionActivityDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails">
        <link:definition>2458430 - Disclosure - Share-based awards - Summary of Stock Option Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsNarrativeDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsNarrativeDetails">
        <link:definition>2459431 - Disclosure - Share-based awards - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofWeightedAverageFairValuesandAssumptionsUsedDetails">
        <link:definition>2460432 - Disclosure - Share-based awards - Schedule of Weighted Average Fair Values and Assumptions Used (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsSummaryofRSUandPSUActivityDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofRSUandPSUActivityDetails">
        <link:definition>2461433 - Disclosure - Share-based awards - Summary of RSU and PSU Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsScheduleofNoncashStockCompensationExpenseDetails">
        <link:definition>2462434 - Disclosure - Share-based awards - Schedule of Non-cash Stock Compensation Expense (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Sharecapital" roleURI="http://www.iconplc.com/role/Sharecapital">
        <link:definition>2163116 - Disclosure - Share capital</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SharecapitalDetails" roleURI="http://www.iconplc.com/role/SharecapitalDetails">
        <link:definition>2464435 - Disclosure - Share capital (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformation" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformation">
        <link:definition>2165117 - Disclosure - Business Segment and Geographical Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformationTables" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationTables">
        <link:definition>2366313 - Disclosure - Business Segment and Geographical Information (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformationNarrativeDetails" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationNarrativeDetails">
        <link:definition>2467436 - Disclosure - Business Segment and Geographical Information - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofRevenuebyGeographicalAreaDetails">
        <link:definition>2468437 - Disclosure - Business Segment and Geographical Information - Schedule of Distribution of Revenue by Geographical Area (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofIncomefromOperationsbyGeographicalAreaDetails">
        <link:definition>2469438 - Disclosure - Business Segment and Geographical Information - Schedule of Distribution of Income from Operations by Geographical Area (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationScheduleofDistributionofDepreciationandAmortizationbyGeographicalAreaDetails">
        <link:definition>2470439 - Disclosure - Business Segment and Geographical Information - Schedule of Distribution of Depreciation and Amortization by Geographical Area (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="iclr_PaymentsForShareRepurchaseCosts" abstract="false" name="PaymentsForShareRepurchaseCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent" abstract="false" name="IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrent" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember" abstract="true" name="SeniorSecuredCreditFacilityAndSeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_PerformanceShareUnitMember" abstract="true" name="PerformanceShareUnitMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio" abstract="false" name="DebtInstrumentCovenantFirstLienNetLeverageRatio" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="iclr_SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" abstract="false" name="SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="iclr_AdjustmentToRetainedEarningShareRepurchaseCosts" abstract="false" name="AdjustmentToRetainedEarningShareRepurchaseCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_UnrecognizedTaxBenefitsCurrentYearTaxPositions" abstract="false" name="UnrecognizedTaxBenefitsCurrentYearTaxPositions" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_OtherUndenominatedCapitalMember" abstract="true" name="OtherUndenominatedCapitalMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedContractWithCustomerLiabilityCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems" abstract="true" name="CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentAmortizationAfterTax" abstract="false" name="OtherComprehensiveIncomeLossCashFlowHedgeAdjustmentAmortizationAfterTax" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_StockRepurchaseProgramAuthorizedPercentage" abstract="false" name="StockRepurchaseProgramAuthorizedPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_RestructuringReserveReleaseOfAccrual" abstract="false" name="RestructuringReserveReleaseOfAccrual" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_LongTermDebtCurrentMaturitiesGross" abstract="false" name="LongTermDebtCurrentMaturitiesGross" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_A2021RestructuringPlanMember" abstract="true" name="A2021RestructuringPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_SeniorSecuredNotesMember" abstract="true" name="SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_PRAHealthSciencesIncMember" abstract="true" name="PRAHealthSciencesIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_OncacareMember" abstract="true" name="OncacareMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_VariableRateComponentOneMember" abstract="true" name="VariableRateComponentOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld" abstract="false" name="BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="iclr_SeniorSecuredRevolvingCreditFacilityMember" abstract="true" name="SeniorSecuredRevolvingCreditFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrentPercent" abstract="false" name="IncreaseDecreaseInContractWithCustomerAssetGrossCurrentPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_ShceduleOfOperatingIncomeByGeographyTableTextBlock" abstract="false" name="ShceduleOfOperatingIncomeByGeographyTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="iclr_ContractsReceivableGross" abstract="false" name="ContractsReceivableGross" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_VariableRateComponentTwoMember" abstract="true" name="VariableRateComponentTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate" abstract="false" name="DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest" abstract="false" name="IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureTable" abstract="true" name="CompensationRelatedCostsShareBasedPaymentsDisclosureTable" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType"/>
  <xs:element id="iclr_DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage" abstract="false" name="DebtInstrumentBasisSpreadOnVariableRateStepDownPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_EarlyTerminationOptionsReasonablyCertainToBeExercisedRightOfUseAssetObtainedInExchangeForOperatingLeaseLiability" abstract="false" name="EarlyTerminationOptionsReasonablyCertainToBeExercisedRightOfUseAssetObtainedInExchangeForOperatingLeaseLiability" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" abstract="true" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_VariableRateComponentDomain" abstract="true" name="VariableRateComponentDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_CustomerBMember" abstract="true" name="CustomerBMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_ContractsReceivableNet" abstract="false" name="ContractsReceivableNet" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_NonExecutiveDirectorMember" abstract="true" name="NonExecutiveDirectorMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_LongTermDebtMaturityYearFourAndThereafter" abstract="false" name="LongTermDebtMaturityYearFourAndThereafter" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" abstract="false" name="IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_TermLoanFacilityMember" abstract="true" name="TermLoanFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_RestOfWorldMember" abstract="true" name="RestOfWorldMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_EquityMethodInvestmentMajorityOwnershipPercentage" abstract="false" name="EquityMethodInvestmentMajorityOwnershipPercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable" abstract="false" name="ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent" abstract="false" name="IncreaseDecreaseInContractWithCustomerAssetGrossCurrent" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember" abstract="true" name="ConsultantsRestrictedStockUnitsTwentyNineteenMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" abstract="false" name="ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_ScheduleOfNonCashStockCompensationExpenseTableTextBlock" abstract="false" name="ScheduleOfNonCashStockCompensationExpenseTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_AdvancePaymentsNettedAgainstUnbilledContractsReceivable" abstract="false" name="AdvancePaymentsNettedAgainstUnbilledContractsReceivable" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BuybackProgramMember" abstract="true" name="BuybackProgramMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_NonCurrentOperatingLeaseLiabilitiesMember" abstract="true" name="NonCurrentOperatingLeaseLiabilitiesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_CustomerAMember" abstract="true" name="CustomerAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_Clients610Member" abstract="true" name="Clients610Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_LeaseLiabilityMember" abstract="true" name="LeaseLiabilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit" abstract="false" name="IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_OtherCustomersMember" abstract="true" name="OtherCustomersMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_VariableRateComponentAxis" abstract="true" name="VariableRateComponentAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="iclr_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate" abstract="false" name="BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_Clients15Member" abstract="true" name="Clients15Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_OtherUndenominatedCapital" abstract="false" name="OtherUndenominatedCapital" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_PerformanceBasedGrantsMember" abstract="true" name="PerformanceBasedGrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_LongTermDebtExcludingCurrentMaturitiesGross" abstract="false" name="LongTermDebtExcludingCurrentMaturitiesGross" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld" abstract="false" name="BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="iclr_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding" abstract="false" name="OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_Clients1125Member" abstract="true" name="Clients1125Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentOperatingLeaseLiability" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_EquityMethodInvestmentPeriodMajorityVotingShareCapitalCanBeSoldToTheCompany" abstract="false" name="EquityMethodInvestmentPeriodMajorityVotingShareCapitalCanBeSoldToTheCompany" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="iclr_RestOfEuropeMember" abstract="true" name="RestOfEuropeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentOperatingLeaseLiability" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
