The Irish Republican Army was formed in 1919 as the successor to the Irish Volunteers, a militant nationalist group. The IRA played a leading role in the Irish War of Independence from 1919-21. When London negotiated independence for Ireland within the British Empire, the IRA split into two factions - one supporting the settlement, the other opposing it. The first group formed the nucleus of the new Irish Free State Army. The second, known as the ''Irregulars,'' was defeated in another civil war (1922-23) but went underground and engaged in violence. The underground group was declared illegal.
When Ireland gained full independence in the late '40s, the IRA focused on the north, trying to drive out the British by force.
Sectarian attacks by Protestants, mainly in Belfast, in 1969 partly led to the resurgence of militant Republicanism. The old ''official'' IRA was largely defunct and Roman Catholics were defenseless in the early attacks until the British Army came in to reinforce local police. Subsequently the official IRA was usurped by the youthful Provisional wing of the IRA, which is committed to the use of terror tactics.
As government intelligence and countersecurity measures became more effective , the Provisionals regrouped and formed cell structures. Overall IRA strength is unknown publicly, but there are estimated to be 300 to 500 activists, with others providing backup, shelter, and transport.