In May 1915, the IRB Military Council, consisting of Joseph Plunkett and Seán Mac Diarmada as well as Ceannt, began plans for a rebellion. Ceannt was one of the seven men to sign the Proclamation of Independence for the Irish Republic and had been appointed Director of Communications. He was made commandant of the 4th Battalion of the Volunteers, and during the Rising was stationed at the South Dublin Union, with more than 100 men under his command, notably his second-in-command Cathal Brugha, and W. T. Cosgrave. The South Dublin Union controlled a large area south of Kilmainham around Dolphin's Barn.