Under the leadership of De Valera the dissident Sinn Féin group, known as Republicans, called for a resumption of the struggle against Britain. The majority of the IRA were also anti-Treaty, and became known as the Irregulars.

After a six-month period in which positions were consolidated and attempts at reconciliation were made, government troops attacked the headquarters of the Irregulars in the Four Courts building in Dublin in June 1922, initiating the Irish Civil War.

By August of that year the pro-Treatyites, better organised and supported with British aid, had retaken control of all of the country's urban area. There followed a protracted guerrilla campaign by the Irregulars, recalling the tactics used in the Anglo-Irish War of 1919-1921.

However, following the death of the vehement anti-Treaty leader Liam Lynch in April 1923, a ceasefire was called, operative from April 30.

Meanwhile, the Provisional Government, headed by William Thomas Cosgrave after the assassination of Collins in August 1922, drafted a new constitution providing for a bicameral legislature, the Oireachtas, comprising Dáil Éireann (Assembly of Ireland) and Seanad Éireann (the Senate), nominally headed by the British monarch, represented by the Governor-general.

The constitution was ratified by the Dáil in December 1922 and the Provisional Government was dissolved. The official government of the Irish Free State was instituted at once, with Cosgrave assuming office as president of the Executive Council.