Irish Republican Army (1917–22) in the context of "Free State Army"

The Irish Republic (Irish: Poblacht na hÉireann or Saorstát Éireann) was a revolutionary state that declared its independence from the United Kingdom of Great Britain and Ireland in January 1919. The Republic claimed jurisdiction over the whole island of Ireland, but by 1920 its functional control was limited to only 21 of Ireland's 32 counties, and British state forces maintained a presence across much of the north-east, as well as Cork, Dublin and other major towns. The republic was strongest in rural areas, and through its military forces was able to influence the population in urban areas that it did not directly control.

Its origins date back to the Easter Rising of 1916, when Irish republicans seized key locations in Dublin and proclaimed an Irish Republic. The rebellion was crushed, but the survivors united under a reformed Sinn Féin party to campaign for a republic. In the 1918 United Kingdom general election, the party won a clear majority of the constituencies in Ireland; in many seats Sinn Féin candidates did not face meaningful opposition. The Sinn Féin winners did not take their seats in the U.K. parliament, but instead gathered in Dublin on 21 January 1919 to form the first Dáil (legislature) of Ireland. Republicans then established a government, a court system and a police force. At the same time, the Irish Volunteers, who came under the control of the Dáil and became known as the Irish Republican Army, fought against British state forces in the Irish War of Independence.

The National Army, sometimes unofficially referred to as the Free State Army or the Regulars, was the army of the Irish Free State from January 1922 until October 1924. Its role in this period was defined by its service in the Irish Civil War, in defence of the institutions established by the Anglo-Irish Treaty. Michael Collins was the army's first commander-in-chief until his death in August 1922.

The army made its first public appearance on 31 January 1922, when command of Beggars Bush Barracks was handed over from the British Army. Its first troops were the Pro-Treaty IRA—those volunteers of the Irish Republican Army (IRA) who supported the Anglo-Irish Treaty and the "Provisional Government of Ireland" formed thereunder. Conflict arose between the National Army and those that opposed the government of the Irish Free State namely the anti-Treaty components of the IRA. On 28 June 1922 the National Army commenced an artillery bombardment of anti-Treaty IRA forces who were occupying the Four Courts in Dublin, thus beginning the Irish Civil War.