The term Irish Republican Army was used first by one of the two Fenian wings (The Fenian Brotherhood in America, which was the American side of the Irish Republican Brotherhood, split soon after the American Civil War) during the invasion of Canada by Fenian forces, in 1886. The title Irish Republican Army was also used, in a spoken form, during the 1867 rising in Ireland (organised by the Irish Republican Brotherhood). James Connolly, leader of the Irish Citizens' Army prior to the 1916 rising and commander of all republican forces during the rising ordered that there was no longer 'Volunteers' or 'Citizens' Army' anymore but the Army of the Irish Republic (or Irish Republican Army).

It is wrong opionion that the IRA had been developed from the Irish Republican Brotherhood. The two organisations maintained their separate existence until the extinction of the Irish Republican Brotherhood sometime in the 1930's (or possibly do still maintain their separate existence according to some unsubstantiated rumours). The fact that the IRB was a seperate organisation is illustrated by the decision made by Michael Collins in 1921 that each member could make up his/her own mind regarding the treaty and the suggestion at the end of the Civil War that the IRA surrender its arms to the Irish Republican Brotherhood and so not surrender them to the Irish Free State.

Like Sinn Féin, however, it split over the post-war settlement that established the Irish Free State and brought about the partition of Ireland. A smaller faction, under the guerrilla leader Michael Collins, accepted the settlement and became part of the Army. The larger faction under Eamon De Valera, then called the Irregulars, fought the Irish Free State government in the Irish Civil War of 1922-1923.

Although it surrendered in 1923, the IRA continued to recruit and train members and periodically to engage in acts of sabotage. It was outlawed in 1931 and again in 1935.

After Ireland withdrew from the Commonwealth in 1948, the IRA turned its attention to removing British rule from Ulster (Northern Ireland) and establishing a unified Irish republic. In the 1950s the IRA conducted a limited campaign in the border areas to the north, which included occasional attacks on army barracks and police stations.