He joined the Ancient Order of Hibernians, a Catholic nationalist organisation, and served in the Easter Rising in 1916 in the GPO as an intelligence officer.

Michael Collins had taken a particular interest in the escape, and actually visited Austin Stack in the prison under a false name to finalise the arrangements.

[12] On 26 March 1922, the anti-Treaty officers of the IRA held a convention in Dublin, in which they rejected the Treaty and repudiated the authority of the Dáil.

"[15] On 14 April 1922 O'Connor was one of a number of IRA leaders in a 200-strong force of that occupied the Four Courts building in the centre of Dublin in defiance of the Provisional Government.

They intended to provoke the British troops (who were still in the country) into attacking them, which they thought would restart the war with Britain and re-unite the IRA against their common enemy.

[citation needed] They also occupied other smaller buildings regarded as being associated with the former British administration, such as the Ballast Office and the Freemasons' Hall in Molesworth Street, but the Four Courts remained the focus of interest.

On 22 June 1922 Sir Henry Wilson was assassinated in London by two IRA men, Reginald Dunne and Joseph O'Sullivan, each a former British soldier.

[17] Lloyd George wrote an angry letter to Collins, which included the line "...still less can Mr. Rory O'Connor be permitted to remain his followers and his arsenal in open rebellion in the heart of Dublin... organizing and sending out from this centre enterprises of murder not only in the area of your Government..." On 28 June 1922, after the Four Courts garrison had kidnapped Ginger O'Connell, a general in the National Army, Collins gave orders for the shelling of the Four Courts with borrowed artillery lent by Winston Churchill.

The shelling led to the Four Courts catching fire, damaging parts of the building in addition to destroying numerous government documents.

This incident marked the official start of the Irish Civil War, as fighting broke out openly around the country between pro- and anti-Treaty factions.

[18] On 8 December 1922, along with three other republicans Liam Mellows, Richard Barrett and Joe McKelvey captured with the fall of the Four Courts, Rory O'Connor was executed by firing squad in reprisal for the anti-treaty IRA's killing of Free State TD Seán Hales.

[21] On his execution, the equestrienne Joan de Sales La Terriere, a close friend of O'Connor, named her son in his honour.