Anti-treatyites , having opposed the Treaty in the Dáil , mostly withdrew from the assembly and , having formed an opposition " republican government " under Éamon de Valera , began a political campaign from March 1922 . At the same time the powerful IRA Army Executive divided , and its anti-Treaty members refused to be bound by the Dáil vote that had ratified it . Barracks that were being evacuated by the British army , in line with the Treaty , were sometimes taken over by anti-Treaty forces . The Dunmanway killings in April emphasised the government 's lack of control . In May 1,200 Garda Síochána recruits mutinied . A force led by Rory O'Connor occupied four central buildings in Dublin on 14 April . The Provisional Government ignored this challenge to its authority , hoping that the occupiers would realise that they had achieved nothing , and leave . Instead some incidents at the Four Courts in late June led to the open outbreak of the Irish Civil War on 28 June
Dunmanway killings
Dunmanway killings
Provisional Government of Ireland (1922)