The militant arm of the Sinn Fein party forms the Irish Republican Army (IRA), with a mission to
safeguard the independence of the Irish Republic.
What actually happened in this year was that the Irish Republican Army, which had fought Britain in the Irish War of Independence, split into two factions, on opposite sides of the debate over the treaty which established the Irish Free State. This lead to the Irish Civil War.
The treaty had been negotiated by Michael Collins, the IRA's military commander, and Arthur Griffith, the founder of Sinn Fein. Although it was ratified by Dail Eireann, the independent parliament set up in Ireland following the general election of 1918, many republicans were displeased that it stopped short of effecting a full separation from Britain and its crown. Most of those who had fought the war of independence believed it presented a series of stepping stones to full independence, and set about creating the apparatus of a new state. Others, however, led by Eamon de Valera, again took up arms to fight for a full republic with no ties to Britain.
The IRA, then, was now split into two factions. The first formed the regular Irish army, and supported the government of the Free State, while the second, known as the irregulars, fought to undermine the state, and precipitate a further war with Britain. They were unsuccessful, and the vast majority subsequently reconciled themselves to the new political entity, forming a party called Fianna Fail. Others, however, continued to reject the new state, and remained members of the now proscribed IRA.
The pre-1922 IRA is generally termed the Old IRA, the organisation which fought the war of independence, but there have been many subsequent organisations calling themselves the IRA, with varying objectives and methods. | 
