Before the anti-treaty and pro-treaty split, he was considered closely associated with Michael Collins Deasy was born in Kilmacsimon, Bandon in County Cork on 6 May 1896, and educated in the local school at Ballinadee.
In the months that followed he, along with others like Éamon de Valera and Liam Lynch, tried to persuade Michael Collins to renegotiate aspects of the treaty, especially to remove an oath to the British king from the constitution of the new Irish Free State.
In late July, he commanded 1,500 anti-treaty fighters who held a line around Kilmallock south of Limerick city against about 2,000 Free State troops under Eoin O'Duffy.
Deasy's men were the most experienced IRA fighters of the 1919-21 war and held their position until 8 August, when they were outflanked by seaborne landings on the southern coast.
In August 1922, he was in command of a band of republican guerrillas in west Cork, when they heard that Free State leader Michael Collins was in the area.
In January 1923, by which time he had become Deputy Chief of Staff of the IRA,[3] he was captured by Free State forces near Clonmel and sentenced to death.
On the day that his order was published Free State authorities demanded that the prisoners in a jail in Limerick sign a statement agreeing to unconditional surrender, threatening wholescale executions to those who refused.