Seán MacMahon was an Irish nationalist who fought in the 1916 rising. He took the pro-treaty side in the Irish Civil War (1922–23) and rose to be chief of staff of the Irish Defence Forces

The Chief of Staff of the Defence Forces (COS) is charged with the executive management of the Irish Defence Forces, and is the most senior military officer of the Army, Naval Service and Air Corps branches. The Chief of Staff is appointed by the President of Ireland, who is the Supreme Commander of the Defence Forces, on the recommendation of the Minister for Defence subject to the approval of the Government of Ireland. The office of the Chief of Staff consists of his personal staff, a strategic planning office, a public relations section and the military judge.