On Sunday, 21 November 1920, the Headquarters Intelligence Staff of the IRA and its special Counterintelligence Branch under the leadership of Michael Collins assassinated 14 of Cumming's case officers. Many agents appear to have escaped the IRA execution squads that morning, but Whitehall feared that more of its professional agents would be identified and suffer the same fate; this prompted the hasty withdrawal of most of the remaining SIS agents from Ireland in the days that followed. A blue plaque was unveiled on 30 March 2015 in Cumming's name at the SIS headquarters at 2 Whitehall Court.