The intelligence staff of Michael Collins Irish Republican Army penetrated the unit . Using DMP detectives Ned Broy and David Nelligan , Michael Collins was able to learn the names and lodgings of the M04 ( x ) agents , referred to by IRA operatives as " The Cairo Gang " . On Bloody Sunday , Collins ordered his Counter-intelligence Unit , The Squad , to assassinate 25 M04 ( x ) agents , several British Courts Martial Officers , at least one agent reporting to Basil Thomson , and several intelligence officers attached to the Royal Irish Constabulary Auxiliary Division , at their lodgings throughout Dublin . Although the shooting of 14 British officers had the desired effect on British morale , in many ways Bloody Sunday was a botched job . Three of Collins 's men were apprehended after engaging in a shoot-out on the street , and at least two of the wounded British officers had no connection whatsoever to British Intelligence . Moreover , with MO4 ( x ) having fielded a total of 175 agents of the DDSB , Collins 's operation only temporarily slowed British momentum . Within days , the remaining 160-odd M04 ( x ) agents were re-established in secure quarters inside solidly Loyalist hotels in Dublin , from where they continued to pursue Collins and the IRA relentlessly right up until the Truce . In December 1920 the entire DDSB was transferred from British Army Command to civil command under Deputy Police Commissioner General Ormonde Winter , and thereafter was known as " D Branch " within Dublin Castle . By January 1921 , the highly experienced MI6 operative David Boyle arrived at Dublin Castle to take over the day-to-day management of D Branch . The unit 's former commander , Colonel Wilson , resigned in protest for having had his command taken from him . D Branch thrived under Boyle 's leadership . The net impact of Collins 's strike of Bloody Sunday , November 21 , 1920 , was therefore quite negligible — even though the IRA had not gone up against MI5 professionals but instead only a quickly trained outfit of amateur army " D-Listers . " That afternoon , a mixed force of the British Army , the Royal Irish Constabulary , and the Black and Tans retaliated by indiscriminately shooting dead 14 civilians at a Gaelic Football match at Croke Park
Bloody Sunday
Bloody Sunday (1920)
MI5