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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:d58a62c4-d45d-4e8e-8bae-d0598a484b89,g:7caf3bbf-8496-4655-ad32-719e0850f6f7-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:iclr="http://www.iconplc.com/20240630" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types1="http://www.xbrl.org/dtr/type/2020-01-21" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2022-03-31" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.iconplc.com/20240630">
  <xs:import namespace="http://fasb.org/srt/2024" schemaLocation="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2024" schemaLocation="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.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://www.xbrl.org/dtr/type/2022-03-31" schemaLocation="https://www.xbrl.org/dtr/type/2022-03-31/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/2024" schemaLocation="https://xbrl.sec.gov/country/2024/country-2024.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2024" schemaLocation="https://xbrl.sec.gov/dei/2024/dei-2024.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-20240630_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-20240630_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" 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-20240630_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-20240630_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="Cover" roleURI="http://www.iconplc.com/role/Cover">
        <link:definition>0000001 - 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>9952151 - 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>9952152 - 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>9952153 - 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>9952154 - 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>9952155 - 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>9952156 - 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>9952157 - 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="Summaryofsignificantaccountingpolicies" roleURI="http://www.iconplc.com/role/Summaryofsignificantaccountingpolicies">
        <link:definition>9952158 - Disclosure - Summary of 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="Revenue" roleURI="http://www.iconplc.com/role/Revenue">
        <link:definition>9952159 - 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="Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities" roleURI="http://www.iconplc.com/role/Accountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilities">
        <link:definition>9952160 - 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="Goodwill" roleURI="http://www.iconplc.com/role/Goodwill">
        <link:definition>9952161 - 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="Intangibleassets" roleURI="http://www.iconplc.com/role/Intangibleassets">
        <link:definition>9952162 - 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="Fairvaluemeasurements" roleURI="http://www.iconplc.com/role/Fairvaluemeasurements">
        <link:definition>9952163 - 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="Restructuring" roleURI="http://www.iconplc.com/role/Restructuring">
        <link:definition>9952164 - 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="Operatingleases" roleURI="http://www.iconplc.com/role/Operatingleases">
        <link:definition>9952165 - 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="Bankcreditlinesloanfacilitiesandnotes" roleURI="http://www.iconplc.com/role/Bankcreditlinesloanfacilitiesandnotes">
        <link:definition>9952166 - Disclosure - Bank credit lines, loan facilities and notes</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="Derivatives" roleURI="http://www.iconplc.com/role/Derivatives">
        <link:definition>9952167 - Disclosure - Derivatives</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>9952168 - 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="Netincomeperordinaryshare" roleURI="http://www.iconplc.com/role/Netincomeperordinaryshare">
        <link:definition>9952169 - 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="Sharebasedawards" roleURI="http://www.iconplc.com/role/Sharebasedawards">
        <link:definition>9952170 - 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="Sharecapital" roleURI="http://www.iconplc.com/role/Sharecapital">
        <link:definition>9952171 - 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="BusinessSegmentandGeographicalInformation" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformation">
        <link:definition>9952172 - 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="Subsequentevents" roleURI="http://www.iconplc.com/role/Subsequentevents">
        <link:definition>9952173 - Disclosure - Subsequent events</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="SummaryofsignificantaccountingpoliciesPolicies" roleURI="http://www.iconplc.com/role/SummaryofsignificantaccountingpoliciesPolicies">
        <link:definition>9954471 - Disclosure - Summary of 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="SummaryofsignificantaccountingpoliciesTables" roleURI="http://www.iconplc.com/role/SummaryofsignificantaccountingpoliciesTables">
        <link:definition>9954472 - Disclosure - Summary of significant accounting policies (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="RevenueTables" roleURI="http://www.iconplc.com/role/RevenueTables">
        <link:definition>9954473 - 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="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesTables">
        <link:definition>9954474 - 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="GoodwillTables" roleURI="http://www.iconplc.com/role/GoodwillTables">
        <link:definition>9954475 - 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="IntangibleassetsTables" roleURI="http://www.iconplc.com/role/IntangibleassetsTables">
        <link:definition>9954476 - 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="FairvaluemeasurementsTables" roleURI="http://www.iconplc.com/role/FairvaluemeasurementsTables">
        <link:definition>9954477 - Disclosure - Fair value measurements (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="RestructuringTables" roleURI="http://www.iconplc.com/role/RestructuringTables">
        <link:definition>9954478 - 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="OperatingleasesTables" roleURI="http://www.iconplc.com/role/OperatingleasesTables">
        <link:definition>9954479 - 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="BankcreditlinesloanfacilitiesandnotesTables" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesTables">
        <link:definition>9954480 - Disclosure - Bank credit lines, loan facilities and notes (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="DerivativesTables" roleURI="http://www.iconplc.com/role/DerivativesTables">
        <link:definition>9954481 - Disclosure - Derivatives (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="IncometaxesTables" roleURI="http://www.iconplc.com/role/IncometaxesTables">
        <link:definition>9954482 - 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="NetincomeperordinaryshareTables" roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareTables">
        <link:definition>9954483 - 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="SharebasedawardsTables" roleURI="http://www.iconplc.com/role/SharebasedawardsTables">
        <link:definition>9954484 - 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="BusinessSegmentandGeographicalInformationTables" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationTables">
        <link:definition>9954485 - 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="SummaryofsignificantaccountingpoliciesScheduleofFiniteLivedIntangibleAssetsDetails" roleURI="http://www.iconplc.com/role/SummaryofsignificantaccountingpoliciesScheduleofFiniteLivedIntangibleAssetsDetails">
        <link:definition>9954486 - Disclosure - Summary of significant accounting policies - Schedule of Finite-Lived 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="RevenueScheduleofDisaggregationofRevenueDetails" roleURI="http://www.iconplc.com/role/RevenueScheduleofDisaggregationofRevenueDetails">
        <link:definition>9954487 - Disclosure - Revenue - Schedule of Disaggregation of 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="AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofAccountsReceivablesandUnbilledRevenueDetails" roleURI="http://www.iconplc.com/role/AccountsreceivableunbilledrevenuecontractassetsandunearnedrevenueorpaymentsonaccountcontractliabilitiesScheduleofAccountsReceivablesandUnbilledRevenueDetails">
        <link:definition>9954488 - 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>9954489 - 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>9954490 - 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>9954490 - 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="GoodwillScheduleofGoodwillDetails" roleURI="http://www.iconplc.com/role/GoodwillScheduleofGoodwillDetails">
        <link:definition>9954491 - 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>9954492 - 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="IntangibleassetsScheduleofFiniteLivedIntangibleAssetsDetails" roleURI="http://www.iconplc.com/role/IntangibleassetsScheduleofFiniteLivedIntangibleAssetsDetails">
        <link:definition>9954493 - Disclosure - Intangible assets - Schedule of Finite-Lived 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="IntangibleassetsNarrativeDetails" roleURI="http://www.iconplc.com/role/IntangibleassetsNarrativeDetails">
        <link:definition>9954494 - Disclosure - Intangible assets - 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="FairvaluemeasurementsMeasurementofFairValueofMajorAssetandLiabilityclassedDetails" roleURI="http://www.iconplc.com/role/FairvaluemeasurementsMeasurementofFairValueofMajorAssetandLiabilityclassedDetails">
        <link:definition>9954495 - Disclosure - Fair value measurements - Measurement of Fair Value of Major Asset and Liability classed (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="FairvaluemeasurementsNarrativeDetails" roleURI="http://www.iconplc.com/role/FairvaluemeasurementsNarrativeDetails">
        <link:definition>9954496 - Disclosure - Fair value measurements - 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="RestructuringNarrativeDetails" roleURI="http://www.iconplc.com/role/RestructuringNarrativeDetails">
        <link:definition>9954497 - 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>9954498 - 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>9954499 - 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="OperatingleasesScheduleofLeaseCostsDetails" roleURI="http://www.iconplc.com/role/OperatingleasesScheduleofLeaseCostsDetails">
        <link:definition>9954500 - 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>9954501 - 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>9954502 - 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>9954502 - 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="BankcreditlinesloanfacilitiesandnotesScheduleofLongtermDebtInstrumentsDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesScheduleofLongtermDebtInstrumentsDetails">
        <link:definition>9954503 - Disclosure - Bank credit lines, loan facilities and notes - Schedule of Long-term Debt Instruments (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="BankcreditlinesloanfacilitiesandnotesScheduleofContractualMaturitiesofDebtDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesScheduleofContractualMaturitiesofDebtDetails">
        <link:definition>9954504 - Disclosure - Bank credit lines, loan facilities and notes - 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="BankcreditlinesloanfacilitiesandnotesNarrativeDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesNarrativeDetails">
        <link:definition>9954505 - Disclosure - Bank credit lines, loan facilities and notes - 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="BankcreditlinesloanfacilitiesandnotesScheduleofPrincipalPaymentsonDebtInstrumentsDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesScheduleofPrincipalPaymentsonDebtInstrumentsDetails">
        <link:definition>9954506 - Disclosure - Bank credit lines, loan facilities and notes - Schedule of Principal Payments on Debt Instruments (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="BankcreditlinesloanfacilitiesandnotesScheduleofMovementonDebtInstrumentDetails" roleURI="http://www.iconplc.com/role/BankcreditlinesloanfacilitiesandnotesScheduleofMovementonDebtInstrumentDetails">
        <link:definition>9954507 - Disclosure - Bank credit lines, loan facilities and notes - Schedule of Movement on Debt Instrument (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="DerivativesNarrativeDetails" roleURI="http://www.iconplc.com/role/DerivativesNarrativeDetails">
        <link:definition>9954508 - Disclosure - Derivatives - 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="DerivativesEffectofDerivativeFinancialInstrumentsonStatementofFinancialPositionDetails" roleURI="http://www.iconplc.com/role/DerivativesEffectofDerivativeFinancialInstrumentsonStatementofFinancialPositionDetails">
        <link:definition>9954509 - Disclosure - Derivatives - Effect of Derivative Financial Instruments on Statement of Financial Position (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="IncometaxesScheduleofComponentsofTaxDetails" roleURI="http://www.iconplc.com/role/IncometaxesScheduleofComponentsofTaxDetails">
        <link:definition>9954510 - Disclosure - Income taxes - Schedule of Components of Tax (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>9954511 - 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="NetincomeperordinaryshareScheduleofReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails" roleURI="http://www.iconplc.com/role/NetincomeperordinaryshareScheduleofReconciliationofNumberofSharesUsedinComputationofBasicandDilutedNetIncomePerOrdinaryShareDetails">
        <link:definition>9954512 - 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>9954513 - 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="SharebasedawardsSummaryofStockOptionActivityDetails" roleURI="http://www.iconplc.com/role/SharebasedawardsSummaryofStockOptionActivityDetails">
        <link:definition>9954514 - 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>9954515 - 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>9954516 - 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>9954517 - 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>9954518 - 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="SharecapitalDetails" roleURI="http://www.iconplc.com/role/SharecapitalDetails">
        <link:definition>9954519 - 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="BusinessSegmentandGeographicalInformationNarrativeDetails" roleURI="http://www.iconplc.com/role/BusinessSegmentandGeographicalInformationNarrativeDetails">
        <link:definition>9954520 - 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>9954521 - 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>9954522 - 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>9954523 - 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_CompensationRelatedCostsShareBasedPaymentsDisclosureTable" abstract="true" name="CompensationRelatedCostsShareBasedPaymentsDisclosureTable" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType"/>
  <xs:element id="iclr_Clients15Member" abstract="true" name="Clients15Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1: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_PerformanceBasedGrantsMember" abstract="true" name="PerformanceBasedGrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_ScheduleOfNonCashStockCompensationExpenseTableTextBlock" abstract="false" name="ScheduleOfNonCashStockCompensationExpenseTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="iclr_Clients610Member" abstract="true" name="Clients610Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems" abstract="true" name="CompensationRelatedCostsShareBasedPaymentsDisclosureLineItems" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" abstract="false" name="SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="iclr_ConsultantsRestrictedStockUnitsTwentyNineteenMember" abstract="true" name="ConsultantsRestrictedStockUnitsTwentyNineteenMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <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_ContractsReceivableGross" abstract="false" name="ContractsReceivableGross" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_VariableRateComponentDomain" abstract="true" name="VariableRateComponentDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_HumanFirstIncMember" abstract="true" name="HumanFirstIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <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_LongTermDebtExcludingCurrentMaturitiesGross" abstract="false" name="LongTermDebtExcludingCurrentMaturitiesGross" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable" abstract="false" name="ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivable" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" abstract="false" name="ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="iclr_RestOfWorldMember" abstract="true" name="RestOfWorldMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_RestOfEuropeMember" abstract="true" name="RestOfEuropeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_TermLoanFacilityMember" abstract="true" name="TermLoanFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" abstract="true" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_ScheduleOfPrincipalPaymentsOnDebtInstrumentTableTextBlock" abstract="false" name="ScheduleOfPrincipalPaymentsOnDebtInstrumentTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="iclr_ContractAssetsAbstract" abstract="true" name="ContractAssetsAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_BuybackProgramMember" abstract="true" name="BuybackProgramMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1: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_FacilityLiabilityMember" abstract="true" name="FacilityLiabilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_OtherCustomersMember" abstract="true" name="OtherCustomersMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <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_ContractsReceivableNet" abstract="false" name="ContractsReceivableNet" 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-types1:domainItemType"/>
  <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_IncreaseDecreaseInContractWithCustomerAssetGrossCurrent" abstract="false" name="IncreaseDecreaseInContractWithCustomerAssetGrossCurrent" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" 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-types1:domainItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerAssetGrossCurrentPercent" abstract="false" name="IncreaseDecreaseInContractWithCustomerAssetGrossCurrentPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="iclr_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" abstract="false" name="IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="iclr_RestrictedStockUnitsTwentyThirteenMember" abstract="true" name="RestrictedStockUnitsTwentyThirteenMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_SeniorSecuredNotesDueMay2029Member" abstract="true" name="SeniorSecuredNotesDueMay2029Member" 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-types1:domainItemType"/>
  <xs:element id="iclr_VariableRateComponentOneMember" abstract="true" name="VariableRateComponentOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_SeniorSecuredRevolvingLoanFacilityMember" abstract="true" name="SeniorSecuredRevolvingLoanFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_SeniorSecuredNotesDueJuly2026Member" abstract="true" name="SeniorSecuredNotesDueJuly2026Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_EarningsPerShareBasicAndDilutedEPSAbstract" abstract="true" name="EarningsPerShareBasicAndDilutedEPSAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="iclr_PersonnelRelatedLiabilitiesMember" abstract="true" name="PersonnelRelatedLiabilitiesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_ScheduleOfOperatingIncomeByGeographyTableTextBlock" abstract="false" name="ScheduleOfOperatingIncomeByGeographyTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <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_DebtInstrumentDiscountRate" abstract="false" name="DebtInstrumentDiscountRate" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <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_ImpairmentOperatingRightOfUseAssetsAndRelatedPropertyPlantAndEquipment" abstract="false" name="ImpairmentOperatingRightOfUseAssetsAndRelatedPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_SeniorSecuredNotesDueMay2034Member" abstract="true" name="SeniorSecuredNotesDueMay2034Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_NonExecutiveDirectorMember" abstract="true" name="NonExecutiveDirectorMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_SeniorSecuredNotesDueMay2027Member" abstract="true" name="SeniorSecuredNotesDueMay2027Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="iclr_StockRepurchaseProgramAuthorizedPercentage" abstract="false" name="StockRepurchaseProgramAuthorizedPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="iclr_InterestRateCapAndInterestRateSwapMember" abstract="true" name="InterestRateCapAndInterestRateSwapMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_Clients1125Member" abstract="true" name="Clients1125Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="iclr_DerivativeVariableInterestRateReferenceRateInExcessOf" abstract="false" name="DerivativeVariableInterestRateReferenceRateInExcessOf" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
</xs:schema>
