[1] Its first troops were those volunteers of the Irish Republican Army (IRA) who supported the Anglo-Irish Treaty and the "Provisional Government of Ireland" formed thereunder.

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.

The National Army was greatly expanded in size to fight the civil war against the anti-Treaty IRA, in a mostly counter-insurgency campaign that was brought to a successful conclusion in May 1923.

Michael Collins envisaged the new army being built around the pre-existing IRA, but over half of this organisation rejected the compromises made in the Anglo-Irish Treaty, and favoured upholding the revolutionary Irish Republic that had existed from 1919 until 1921.

[citation needed] A force of 4,000 troops was envisaged, but with the impending Civil War, on 5 July 1922 the Provisional Government authorised raising an establishment of 35,000 men.

[4] In March 1922, there was a major stand-off between up to 700 National Army and anti-treaty IRA in Limerick over who would occupy the military barracks being vacated by departing British troops.

[8] The IRA contingent in the Four Courts, who had only small arms, surrendered after two days of shelling and the buildings were stormed by National Army troops.

[9] Michael Collins, Richard Mulcahy and Eoin O'Duffy planned a nationwide offensive, sending columns overland to take Limerick and Waterford and seaborne forces to Counties Cork, Kerry and Mayo.

Collins was killed in an ambush by IRA forces at Béal na Bláth in County Cork on 22 August 1922; General Richard Mulcahy then took command.

[11] The Guard acted, particularly in County Kerry, which they occupied after a successful assault on Tralee in August 1922, with fearsome brutality, beginning the summary execution of captured IRA soldiers.

The most notorious example of this occurred at Ballyseedy where nine IRA prisoners were tied to a landmine; the detonation killed eight and only left one, Stephen Fuller, who was blown clear by the blast to escape.

With the end of the Civil War, the National Army had grown too big for a peacetime role and was too expensive for the new Irish state to maintain.

This situation evolved into what became called the "Army Mutiny",[18] which, after an ultimatum, was resolved relatively peaceably with recognition of the authority of the Irish Free State's Government.