The 4th Battalion of the IRA's First Cork Brigade was based around the towns of Midleton, Youghal and Cobh in East Cork. Under Diarmuid O'Hurley it had been a notably successful unit. The battalion had captured three Royal Irish Constabulary barracks and executed an ambush in Midleton itself - the kind of operations that built reputations and gave volunteers a sense of momentum. By January 1921 they had taken over a disused farmhouse on a hillside overlooking the village of Clonmult. O'Hurley was planning an ambush on a military train at Cobh Junction for 22 February. Two days before that planned operation, they were betrayed. The historian Peter Hart later wrote that the unit 'had become over-confident and fallen into a traceable routine.' An intelligence officer of the Hampshire Regiment found their billet.

British troops of the 2nd Battalion, Hampshire Regiment, under Lieutenant A. R. Koe, surrounded the farmhouse. Two volunteers on lookout spotted them and opened fire. Both were killed immediately, but the shooting alerted the men inside. Jack O'Connell of the Cobh Company, acting as commander on the spot, ordered a sortie - a breakout attempt to reach the local IRA company for reinforcements. O'Connell got through. Three others were killed trying. The volunteers still inside the house attempted to squeeze out through a narrow gable opening; that escape also failed. By the time O'Connell could organise help, British reinforcements had arrived first - regular RIC, the Black and Tans, and the Auxiliaries. The police brought petrol. An Army officer set the thatched roof alight.

With the farmhouse burning around them, the volunteers tried to surrender. What happened next is one of the most contested moments of the War of Independence. The IRA's General Headquarters said some volunteers came out with their hands up while others kept firing. The local RIC county inspector wrote in his confidential monthly report that the surrender was a ruse - that men came out claiming surrender while comrades inside opened fire on the police. The survivors told a different story. Patrick Higgins, who lived through it, gave testimony that lodged in the historical record: 'We were lined up alongside an outhouse with our hands up. The Tans came along and shot every man with the exception of three... A Tan put his revolver to my mouth and fired. Only for the military officer coming along, I would be gone.' Higgins was wounded in the jaw. He survived because an Army officer - not the police - intervened.

Historians remain divided. Peter Hart wrote that 'the Irish survivors testified convincingly that there had been no treachery on their part.' He compared Clonmult to the Kilmichael ambush three months earlier, where the IRA had killed surrendering Auxiliaries - calling Clonmult 'Kilmichael in reverse.' Tom O'Neill, in The Battle of Clonmult: The IRA's Worst Defeat, suggested the police may have heard the volunteers' own ammunition cooking off inside the burning house and interpreted the shots as treachery from comrades still inside. The other possibility, O'Neill notes, is that the British simply lied afterwards to cover what they had done. Twelve IRA volunteers died in the action. Two of the four prisoners taken, Maurice Moore and Paddy O'Sullivan, were executed in Cork military barracks on 28 April. Patrick Higgins was sentenced to death but was reprieved when the truce ended the war on 11 July.

Fourteen IRA members died in connection with Clonmult - the twelve killed at the farmhouse and the two executed afterwards. Add two Black and Tans killed in related actions and six men shot by the IRA in the following days as suspected informers, and the death toll reaches 22. The IRA was certain someone had betrayed the column's location, and the reprisal killings happened fast. Mick Leahy, a local IRA officer, said that 'things went to hell in the battalion' after Clonmult. Diarmuid O'Hurley, the commander who had not been at the farmhouse, was killed on 28 May 1921. The Hampshire Regiment's own historian, Scott Daniell, later wrote that 'like all the Irish operations, it was hateful to the British troops.' What is left at the site today is the rebuilt farmhouse and a memorial. Each year the names are read aloud. They were young men. Most were in their twenties. Several had not yet had time to learn what it was they had agreed to die for.