The day began with an Irish Republican Army (IRA) operation, organised by Michael Collins, to assassinate the "Cairo Gang" – a group of undercover British intelligence agents working and living in Dublin.

That evening, two Irish republicans (Dick McKee and Peadar Clancy) who had helped plan the earlier assassinations, along with a civilian (Conor Clune) who happened to be caught with the others, were beaten and shot dead in Dublin Castle by their British captors, who said that they were killed during an escape attempt.

[8] The events on the morning of 21 November were an effort by the IRA in Dublin, under Michael Collins and Richard Mulcahy, to destroy the British intelligence network in the city.

"The Twelve Apostles"), who were tasked with assassinating prominent RIC officers and British agents, including suspected informers.

On 10 November, Mulcahy narrowly evaded capture in a raid, but British forces seized documents which included names and addresses of 200 IRA members.

[15] Shortly after, Collins ordered the assassination of British agents in the city, judging that if this was not done, the IRA's organisation in the capital would be in grave danger.

Collins's plan had initially been to kill more than 50 suspected British intelligence officers and informers, but the list was reduced to thirty-five on the insistence of Cathal Brugha, the Minister for Defence for the Irish Republic, reportedly on the grounds that there was insufficient evidence against some of those named.

They were captured by an IRA team on Mount Street Bridge and marched to a house on Northumberland Road where they were interrogated and shot dead.

[33] At 28 Earlsfort Terrace, an RIC sergeant named Fitzgerald was killed, but apparently the target was a British lieutenant-colonel Fitzpatrick.

In a 1972 article, historian Tom Bowden concluded that "the officers shot by the IRA were, in the main, involved in some aspect of British intelligence".

[35] Charles Townshend disagreed: in a response published in 1979, he criticised Bowden's work, while presenting evidence from the Collins Papers to show that "several of the 21st November cases were just regular officers".

One of the two men shot at the Gresham Hotel (Wilde) was probably on secret service, but the other was an innocent civilian, killed because the assassins went to the wrong room.

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.

However, for some reason, shots were fired by police as soon as they reached the southwest gate at the Royal Canal end of Croke Park, at 3:25 pm.

[49] The police in the convoy's leading trucks appear to have jumped out, run down the passage to the Canal end gate, forced their way through the turnstiles, and started firing rapidly with rifles and revolvers.

Armed and uniformed men were seen entering the field, and immediately after the firing broke out scenes of the wildest confusion took place.

At the other side of the Park, 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.

[44] List of the Croke Park victims[61] Later that night, two high-ranking IRA officers, Dick McKee and Peadar Clancy, together with another man, Conor Clune, were killed while being held and interrogated in Dublin Castle.

[65] Medical examination found broken bones and abrasions consistent with prolonged assaults, and bullet wounds to the head and body.

"[66] However, Clune's employer, Edward MacLysaght, who viewed the corpses at King George V Hospital, stated that the claim "that their faces were so battered about as to be unrecognisable and horrible to look at is quite untrue.

He found numerous bullet wounds as did a private doctor hired by Edward MacLysaght but no signs of any other injuries such as bayoneting.

[70] Head of British Intelligence Brigadier General Ormonde Winter carried out his own private investigation, interviewing the guards and inspecting the scene, pronouncing himself happy with their account, noting "One of the rebels was lying on his back near the fireplace, with a grenade in his right hand, and the other two were close by.

I extracted the bullet from the door and at once reported to Sir John Anderson who, somewhat dubious of the accuracy of my information, accompanied me to the guardroom.

Some contemporary newspapers, including the nationalist Freeman's Journal, likened the shootings in Croke Park to the Amritsar massacre, which had taken place in India in April 1919.

[73] When Joseph Devlin, an Irish Parliamentary Party Member of Parliament (MP), tried to bring up the Croke Park massacre at Westminster, he was shouted down and physically assaulted by his fellow MPs;[41] the sitting had to be suspended.

Major General Boyd, the British officer commanding Dublin District, added that in his opinion, the firing on the crowd "was indiscriminate, and unjustifiable, with the exception of any shooting which took place inside the enclosure".

[82] The trial for the Lower Mount Street killings was held as a Field General Court-martial at City Hall in Dublin, on Tuesday 25 January 1921.

[86]Mr "C"[87] was brought forward as a witness on 28 January and was identified as the man sleeping in the same bed who escaped by jumping out the window when the attackers came into the room.

[90] The Irish Times reported that on 6 March 1921, the death sentences of Conway and Potter were commuted by the Viceroy of Ireland to penal servitude.