The Anti-Treaty Irish Republican Army was an armed guerilla organization that existed since the split in the IRA over the Anglo-Irish treaty in 1922. Anti-Treaty Sinn Féin, which similarly to the Anti-Treaty IRA emerged in the split over the treaty, served as its political wing, though it was at times disconnected from the Anti-Treaty IRA. The two organizations were politically broad, serving as an umbrella group for Republicans opposed to the treaty, including Socialists and Communists as well as more conservative Catholic nationalists.
It was involved in several armed campaigns including the Irish Civil War between 1922 and 1923, a Sabotage Campaign commonly called the S-plan between 1939 and 1942, a campaign in the north between 1940 and 1942 and the IRA Border Campaign between from 1956 to 1962.
The beginning of the Troubles was the declaration of war against the Anti-Treaty IRA in 1966. The Anti-Treaty IRA was involved in the conflict from 1966 to 1969, though at this time the militant Republican movement had little popular support and few weapons and volunteers. In 1969 the Anti-Treaty IRA and Anti-Treaty Sinn Féin split between the Official Irish Republican Army and the Provisional Irish Republican Army over the issue of abstentionism.