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  <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_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentPortionOfOperatingLeaseLiabilities" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNonCurrentPortionOfOperatingLeaseLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsReceivableAndUnbilledRevenue" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccountsReceivableAndUnbilledRevenue" 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_IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit" abstract="false" name="IncomeTaxExpenseBenefitExcludingOtherTaxExpenseBenefit" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" 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_SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" abstract="false" name="SummaryOfShareBasedCompensationRestrictedStockUnitsPerformanceStockUnitsAwardActivityTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="iclr_DebtInstrumentCovenantFirstLienNetLeverageRatio" abstract="false" name="DebtInstrumentCovenantFirstLienNetLeverageRatio" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <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_TermLoanFacilityMember" abstract="true" name="TermLoanFacilityMember" 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_BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate" abstract="false" name="BusinessCombinationConsiderationTransferredPaymentsToSettleDebtObligationsThatExistedPriorToAcquisitionDate" nillable="true" xbrli:periodType="duration" 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="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssets" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" 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_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset" abstract="false" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <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_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld" abstract="false" name="BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <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_IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" abstract="false" name="IncreaseDecreaseInContractWithCustomerLiabilityGrossCurrentPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <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_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding" abstract="false" name="OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxLongTermFunding" 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_AdvancePaymentsNettedAgainstUnbilledContractsReceivable" abstract="false" name="AdvancePaymentsNettedAgainstUnbilledContractsReceivable" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAssets" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOperatingLeaseRightOfUseAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" 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_ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" abstract="false" name="ChangeInAdvancePaymentsNettedAgainstUnbilledContractsReceivablePercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxAssets" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentDeferredTaxAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="iclr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpensesAndOtherCurrentLiabilities" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentAccruedExpensesAndOtherCurrentLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" 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_LongTermDebtCurrentMaturitiesGross" abstract="false" name="LongTermDebtCurrentMaturitiesGross" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <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_FacilityLiabilityMember" abstract="true" name="FacilityLiabilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types: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_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_CustomerBMember" abstract="true" name="CustomerBMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types: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_OtherLiabilityMember" abstract="true" name="OtherLiabilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <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_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentUnearnedRevenue" abstract="false" name="BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentUnearnedRevenue" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <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_ConsultantsRestrictedStockUnitsTwentyNineteenMember" abstract="true" name="ConsultantsRestrictedStockUnitsTwentyNineteenMember" 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_DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate" abstract="false" name="DebtInstrumentIncreaseDecreaseBasisSpreadOnVariableRate" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="iclr_AdjustmentToRetainedEarningShareRepurchaseCosts" abstract="false" name="AdjustmentToRetainedEarningShareRepurchaseCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
