The killings happened in a tense interval that should have been peace. The Irish War of Independence had been brought to a close by the Anglo-Irish Truce of July 1921. The Anglo-Irish Treaty had been signed in December. A Provisional Government in Dublin was meant to take over from a withdrawing British administration. But the IRA had split. The anti-Treaty side, refusing to accept anything less than a full Republic, controlled most of West Cork; the pro-Treaty side controlled Dublin. British forces had pulled out of West Cork in February 1922, leaving the local IRA effectively the only armed authority on the ground. The local IRA was almost unanimously anti-Treaty and not answering to the new government. And then on 26 March 1922, the anti-Treaty IRA formally repudiated the Provisional Government's authority. The country was sliding toward the Irish Civil War, which would erupt in June. Dunmanway sat in the middle of that lurch.

On the night of 26 April 1922, a group of anti-Treaty IRA men led by Michael O'Neill arrived at the house of Thomas Hornibrook, a former magistrate at Ballygroman near Ballincollig. They wanted to seize his car. Hornibrook had removed the magneto - a key part of the engine - to prevent exactly this kind of theft. With Thomas were his son Samuel and his nephew Herbert Woods, a former British Army captain with a Military Cross. The IRA party tried to force entry. Some of them broke in through a window. Woods shot O'Neill, fatally wounding him. O'Neill's companion Charlie O'Donoghue carried him to a local priest, who pronounced him dead. The next morning O'Donoghue went to Bandon to report. He returned with four armed IRA men. The Hornibrooks and Woods were taken, the house was burned, and according to a letter from a local Protestant woman named Alice Hodder, Herbert Woods was tried by a mock court-martial and hanged. Hodder wrote that O'Neill's brothers had gouged out Woods' eyes while he was still alive. The bodies of the three men have never been recovered. O'Neill was buried as a Republican martyr.

What followed across the next 48 hours was the killing of thirteen Protestant men and one Catholic. Some were named on a list found by the IRA when the Auxiliary Division evacuated their barracks at Dunmanway workhouse - a list that has since been re-examined and does not, in fact, contain any of the murdered men's names. Six were killed as purported informers. Four others were relatives killed in the absence of the target, including the two sixteen-year-olds, Alexander McKinley and Robert Nagle - boys who died because their fathers were not home. Three more were kidnapped and executed in Bandon as revenge for O'Neill's death. The Reverend Ralph Harbord, son of Rev. Richard Harbord (the original target), survived being shot. Across West Cork, Protestant families fled. One Cork correspondent for the Irish Times described boats and trains leaving for England so packed that 'many had neither a handbag nor an overcoat'. Alice Hodder wrote home that for two weeks there was not standing room on any boat or mail train out of Cork.

The killings were immediately condemned by Sinn Féin and the IRA leadership on both sides of the Treaty divide. Arthur Griffith spoke against them in Dublin. The anti-Treaty TD Sean T. O'Kelly associated his side with that condemnation. A general convention of Irish Protestant churches in Dublin issued a statement noting that hostility to Protestants on religious grounds had been 'almost, if not wholly unknown' in the twenty-six counties where they were a minority - choosing words carefully, perhaps, in a moment when the events at Dunmanway clearly contradicted them. Local IRA commanders Tom Barry, Liam Deasy and Sean Moylan - who had been out of the county during the killings - returned and ordered armed guards posted at the homes of remaining Protestant families to prevent more violence. The Belfast News-Letter on 28 April headlined 'Protestants Slain' and spoke of 'a general massacre of Protestants'. The Northern Whig invoked 1641. No member of the IRA ever claimed responsibility. No one was tried. The Provisional Government, busy losing control of its own country, did nothing.

A century on, historians still argue about why this happened. Peter Hart, whose 1998 book The IRA and Its Enemies put the killings back into Irish public debate, called them primarily revenge attacks carried out by 'angry and frightened young men acting on impulse', with the families' Protestant identity as a substantial factor. Other historians - Niall Meehan, Brian P. Murphy - argue the men were killed for being suspected British informers, not for their religion, citing intelligence documents and the political-military context of an IRA that feared British re-occupation. Meda Ryan claims to have seen all thirteen victims' names listed as 'helpful citizens' in Auxiliary documents. John Regan has argued that the killings should be understood as an IRA preemptive move against people believed to have informed during the war. The debate has at times been intense and bitter; some has spilled into Irish national papers. What is not in dispute is the human toll. Francis Fitzmaurice, David Gray, James Buttimer, Robert Howe, John Chinnery, Alexander McKinley, Robert Nagle, John Bradfield, the three Bandon men, Thomas and Samuel Hornibrook, Herbert Woods - they did not come back. Their families left, mostly for England, mostly forever. West Cork's Protestant population, already small, contracted further and never recovered. In 2022, on the centenary, the Bishop of Cork led a memorial service for the dead. Two descendants of victims spoke. The wound is a hundred years old now, and it is still a wound.