The day began with the killing of fourteen of eighteen British agents of the Cairo Gang, or their informants, by the Irish Republican Army. Later that afternoon, British forces opened fire on the crowd at a Gaelic football match in Croke Park in north Dublin, killing 14 civilians. That same evening there were scattered shootings in the city streets, and three Irish prisoners in Dublin Castle were killed by their British captors under suspicious circumstances.
In response to IRA actions, the British Government formed paramilitary forces to augment the RIC, the "Black and Tans" (a nickname arising from their mixture of uniforms), and the Auxiliary Division (generally known as the Auxiliaries or Auxies). The behaviour of both groups immediately became controversial (one major critic was King George V) for their brutality and violence towards not just IRA suspects and prisoners but Irish people in general. In Dublin, the war largely took the form of assassinations and reprisals on either side.
The events on the morning of 21 November were an effort by the IRA in Dublin, under Michael Collins and Richard Mulcahy to wipe out the British intelligence organisation in the city. It was the police that were responsible for the British reprisals on the afternoon of Bloody Sunday.
In November 1920, Collins ordered the assassination of British agents around the city, judging that if they did not do this, the IRA's organisation in the capital would be in grave danger. Dick McKee was put in charge of planning the operation. The addresses of the British agents were discovered from a variety of sources, including sympathetic housemaids, careless talk from some of the British, and an RIC Sergeant based in Donnybrook barracks. On November 20, the assassination teams, which included the Squad and members of the IRA's Dublin Brigade, were briefed on their targets, who included 20 agents at eight different locations in Dublin. Collins' plan had been to kill over 50 British intelligence officers and informers, but the list was reduced to 35 on the insistence of Cathal Brugha, the Irish Minister for Defence, on the grounds that there was insufficient evidence against some of those named.
In all, 14 people were killed and 6 wounded, including suspected agents and those with no connection to politics, and two Auxiliaries. Four of the British casualties were military intelligence officers and another four were Secret Service or MI5 agents. Only one Squad member was captured, Frank Teeling, and he managed to quickly escape from gaol. One more IRA man was slightly wounded in the hand. However, out of the 35 people on Collins' hit list, only about a third had been killed. IRA man and future Irish politician, Todd Andrews recalled later, "the fact is that the majority of the IRA raids were abortive. The men sought were not in their digs or in several cases, the men looking for them bungled their jobs". Nevertheless the action terrified and crippled British intelligence in Ireland, causing many other agents and informers to flee for Dublin Castle, and caused consternation in the British administration.
Collins justified the killings in this way; "My one intention was the destruction of the undesirables who continued to make miserable the lives of ordinary decent citizens. I have proof enough to assure myself of the atrocities which this gang of spies and informers have committed. If I had a second motive it was no more than a feeling such as I would have for a dangerous reptile. By their destruction the very air is made sweeter… For myself, my conscience is clear. There is no crime in detecting in wartime the spy and the informer. They have destroyed without trial. I have paid them back in their own coin.
Meanwhile, outside the Park, unseen by the crowd, British security forces were approaching and preparing to raid the match. A convoy of troops drove in from the northwest, along Clonliffe Road, while a convoy of police and Auxiliaries approached the Park from the south or Canal end. Their orders were to surround the grounds, guard the exits, and search every man in the Park. The authorities later stated that their intention was to announce by megaphone that all males leaving the stadium would be searched and that anyone leaving by other means would be shot. But for some reason, shots were fired as soon as the police convoy reached the stadium, at 3:25 p.m.
Some of the police later claimed that they were fired on first by IRA sentries, but this has never been proven. Correspondents for the Manchester Guardian and Britain's Daily News interviewed eyewitnesses, and concluded that the "IRA sentries" were actually ticket-sellers: "It is the custom at this football ground for tickets to be sold outside the gates by recognised ticket-sellers, who would probably present the appearance of pickets, and would naturally run inside at the approach of a dozen military lorries. No man exposes himself needlessly in Ireland when a military lorry passes by. The police in the convoy's leading cars appear to have jumped out, pursued these men down the passage to the Canal End gate, forced their way through the turnstiles, and started firing rapidly with rifles and revolvers. Ireland's Freeman's Journal reported that, "the spectators were startled by a volley of shots fired from inside the turnstile entrances. Armed and uniformed men were seen entering the field, and immediately after the firing broke out scenes of the wildest confusion took place. The spectators made a rush for the far side of Croke Park and shots were fired over their heads and into the crowd.
The police kept shooting for about ninety seconds: their commander, Major Mills, later admitted that his men were "excited and out of hand. Some police fired into the fleeing crowd from the pitch, while others, outside the Park, opened fire from the Canal Bridge at spectators who climbed over the Canal End Wall trying to escape. At the other end of the Park, the soldiers on Clonliffe Road were startled first by the sound of the fusillade, then by the sight of panicked people fleeing the grounds. As the spectators streamed out, an armoured car on St James Avenue fired its machine guns over the heads of the crowd, trying to halt them.
By the time Major Mills got his men back under control, the police had fired 114 rounds of rifle ammunition, and an unknown amount of revolver ammunition as well, not counting 50 rounds fired from the machine guns in the armoured car outside the Park. Seven people had been shot to death, and five more had been fatally wounded; another two people had been trampled to death by the crowd. The dead included Jeannie Boyle, who had gone to the match with her fiancé and was due to be married five days later, and two boys aged just 10 and 11. Two football players, Michael Hogan and Jim Egan, had been shot; Hogan was killed, but Egan survived, along with dozens of other wounded and injured. The police raiding party suffered no casualties.
Once the firing had been stopped, the security forces searched the remaining men in the crowd before letting them go. The military raiding party recovered one revolver: a local householder testified that a fleeing spectator had thrown it away in his garden. Once the grounds were cleared, the Park was searched for arms: according to Major Mills, none were found.
The actions of the police were officially unauthorised and were greeted with public horror by the Dublin Castle-based British authorities. In an effort to cover up the nature of the behaviour by Crown forces, a press release was issued which claimed:
A number of men came to Dublin on Saturday under the guise of asking to attend a football match between Tipperary and Dublin. But their real intention was to take part in the series of murderous outrages which took place in Dublin that morning. Learning on Saturday that a number of these gunmen were present in Croke Park, the crown forces went to raid the field. It was the original intention that an officer would go to the centre of the field and speaking from a megaphone, invite the assassins to come forward. But on their approach, armed pickets gave warning. Shots were fired to warn the wanted men, who caused a stampede and escaped in the confusion.
The Times which during the war was a pro-Unionist publication, ridiculed Dublin Castle's version of events, as did a British Labour Party delegation visiting Ireland at the time. The British Brigadier Frank Crozier, technically in command that day, later resigned over what he believed was the official condoning of the unjustified actions of the Auxiliaries in Croke Park. One of his officers told him that, "Black and Tans fired into the crowd without any provocation whatsoever".
Two military courts of inquiry into the massacre were held, and one found that "the fire of the RIC was carried out without orders and exceeded the demands of the situation." Major-General Boyd, the officer commanding Dublin District, added that in his opinion, "the firing on the crowd was carried out without orders, was indiscriminate, and unjustifiable, with the exception of any shooting which took place inside the enclosure. The findings of these courts of inquiry were suppressed by the British Government, and have only recently come to light.
A combination of the loss of the Cairo Gang, which devastated British Intelligence in Ireland, and the public relations disaster that was Bloody Sunday severely damaged the cause of British rule in Ireland and increased support for the republican government under Éamon de Valera. The events of Bloody Sunday have survived in public memory. The Gaelic Athletic Association named one of the stands in Croke Park the 'Hogan Stand' in memory of Michael Hogan, the football player killed in the incident.
James "Skankers" Ryan, who had informed on Clancy and McKee, was shot and killed by the IRA in February 1921.
IRA assassinations continued in Dublin for the remainder of the war, in addition to more large scale urban guerrilla actions by the Dublin Brigade. By the Spring of 1921, the British had rebuilt their Intelligence organisation in Dublin, and the IRA were planning another assassination attempt on British agents in the summer of that year. However, these plans were called off because of the Truce that ended the war on July 11, 1921.