Eoin O'Duffy was an Irish revolutionary, soldier, police commissioner and politician. O'Duffy was the leader of the Monaghan Brigade of the Irish Republican Army (IRA) and a prominent figure in the Ulster IRA during the Irish War of Independence. In this capacity, he became Chief of Staff of the IRA in 1922. He accepted the Anglo-Irish Treaty and as a general became Chief of Staff of the National Army in the Irish Civil War, on the pro-Treaty side.

The Irish Republican Army was an Irish republican revolutionary paramilitary organisation. The ancestor of many groups also known as the Irish Republican Army, and distinguished from them as the "Old IRA", it was descended from the Irish Volunteers, an organisation established on 25 November 1913 that staged the Easter Rising in April 1916. In 1919, the Irish Republic that had been proclaimed during the Easter Rising was formally established by an elected assembly, and the Irish Volunteers were recognised by Dáil Éireann as its legitimate army. Thereafter, the IRA waged a guerrilla campaign against the British occupation of Ireland in the 1919–1921 Irish War of Independence.