 When fighting broke out in Dublin, in June 1922, between pro and anti-Treaty forces, Deasy sided with the anti-treaty IRA in the ensuing Irish Civil War, however, he was reluctant to fight his former comrades and voiced the opinion that the fighting should have ended with the Free State seizure of the Four Courts.In late July, he commanded 1500 anti-Treaty fighters who held a line around Kilmallock south of Limerick city against about 2000 Free State troops under Eoin O'Duffy.Deasy's men were the most experienced IRA fighters of the 1919-21 war and they held their position until 8 August, when they were outflanked by seaborne landings on the southern coast of Ireland.