The Royal Irish Constabulary was the police force in Ireland from 1822 until 1922, when all of the country was part of the United Kingdom. A separate civic police force, the unarmed Dublin Metropolitan Police (DMP), patrolled the capital and parts of County Wicklow, while the cities of Derry and Belfast, originally with their own police forces, later had special divisions within the RIC. For most of its history, the ethnic and religious makeup of the RIC broadly matched that of the Irish population, although Anglo-Irish Protestants were overrepresented among its senior officers.

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.