He played an active role in the Easter Rising of 1916, serving under Éamonn Ceannt as a Captain at the South Dublin Union. Following the rebellion, Cosgrave was sentenced to death, but that was later commuted to penal servitude for life and he was interned at Frongoch internment camp, Wales. While imprisoned, he won a seat for Sinn Féin in the Kilkenny City by-election of August 1917. After his victory, he made a speech on the courthouse balcony. In September 1917, he and Michael Collins addressed a crowd in Dunboyne, County Meath, urging people to join the Irish Volunteers.
He played an active role in the Easter Rising of 1916, serving under Éamonn Ceannt as a Captain at the South Dublin Union. Following the rebellion, Cosgrave was sentenced to death, but that was later commuted to penal servitude for life and he was interned at Frongoch internment camp Wales. While imprisoned, he won a seat for Sinn Féin in the Kilkenny City by-election of August 1917. After his victory, he made a speech on the courthouse balcony. In September 1917, he and Michael Collins addressed a crowd in Dunboyne, County Meath, urging people to join the Irish Volunteers.