Éamonn Ceannt's first serious involvement in national politics was in 1907 when he joined Arthur Griffith’s new political party, Sinn Féin, which opposed Home Rule, promoted the concept of national self-reliance, and aimed at national independence. He was eventually elected to the national council of Sinn Féin. It appears that he was sworn into the Irish Republican
Brotherhood in 1912. On the foundation of the Irish Volunteers in
November 1913, Ceannt was elected to the provisional committee, becoming involved in raising finance for the procurement of arms; he was present at both the Howth and Kilcoole importations.
Ceannt became commandant of the 4th Battalion of the Dublin Brigade in March 1915. Soon after he was co-opted to the IRB Military Council.
Many of the Military Council meetings took place at his home in Dolphin’s Barn in Dublin.
On Easter Monday 1916, Ceannt and 120 men of the 4th Battalion of the Irish Volunteers, occupied the South Dublin Union, a workhouse and hospital spread over fifty-two acres off James’s Street. They held part of the complex until they were informed of the general surrender the following Sunday, 30 April 1916.