O’Duffy does an apprenticeship as an engineer in Wexford before working as an engineer and architect in Monaghan. In 1919 he becomes an auctioneer. He is a leading member of the Gaelic Athletic Association in Ulster in the 1910s. In 1912 he is appointed secretary of the Ulster provincial council. He is also a member of Harps’ Gaelic Football Club.
O’Duffy joins the Irish Republican Army (IRA) in 1917 and is the leader of the Monaghan Brigade of the IRA during the Irish War of Independence and in this capacity becomes Chief of Staff of the IRA in 1922. He is one of the Irish activists who along with Michael Collins accepts the Anglo-Irish Treaty and fights as a general in the Irish Civil War on the pro-Treaty side.
Professionally, O’Duffy becomes the second Commissioner of the Garda Síochána, the police force of the new Irish Free State, after the Civic Guard Mutiny and the subsequent resignation of Michael Staines. He holds this post until 1933, when he is dismissed by Éamon de Valera. In his political life O’Duffy is an early member of Sinn Féin, founded by Arthur Griffith. He is elected as a Teachta Dála (TD) for his home county of Monaghan during the 1921 election.
After a split in 1923 he becomes associated with Cumann na nGaedheal and becomes head of a veterans group then known as the Army Comrades Association. O’Duffy changes its name to “National Guard” and begins to stage fascist-style rallies and adopts a fascist salute. Its members begin to wear blue uniform shirts and become known as the Blueshirts. When government opposition groups form Fine Gael in September 1933, he becomes its first president, reaching the apex of his political power.
Subsequently, the government bans O’Duffy’s National Guard, as well as the group he creates to replace it, the Young Ireland Association, which he in turn replaces with the League of Youth, but their blue shirts indicate its continued fascist ideology. Fine Gael’s other leaders soon tire of his inflammatory rhetoric and the frequent violent behavior of the Blueshirts, but are still surprised when their opposition causes him to resign his party leadership in September 1934. He is then ousted as leader of the Blueshirts as well, but does retain a small loyal following.
An anti-communist, O’Duffy is attracted to the various authoritarian nationalist movements on the Continent. In 1936, he raises the Irish Brigade to fight for Francisco Franco during the Spanish Civil War as an act of Catholic solidarity and is inspired by Benito Mussolini‘s Italy to found the National Corporate Party. He offers to Nazi Germany the prospect of raising an Irish Brigade to fight in Operation Barbarossa during World War II on the Eastern Front against the Soviet Union, but this is not taken up.
Eoin O’Duffy takes no further part in Irish politics and dies in Dublin on November 30, 1944. In spite of his later politics, he is given a state funeral for his earlier contributions to the Irish government. He is buried in Dublin’s Glasnevin Cemetery.