Fighting in the Irish Civil War started on 28 June 1922 and ended in May 1923 when the pro-treaty Free State forces beat the anti-Treaty IRA . Relations between the new Irish government , which was backed by most of the DÃ il and the electorate , and the anti-Treatyites under the nominal leadership of de Valera , now descended into the Irish Civil War ( June 1922 to May 1923 ) , in which the pro-treaty Free State forces defeated the anti-Treaty IRA . Both sides had wanted to avoid civil war , but fighting broke out over the takeover of the Four Courts building in Dublin by anti-Treaty members of the IRA .