Seán Allis Treacy (Irish: Seán Ó Treasaigh; 14 February 1895 – 14 October 1920) was one of the leaders of the Third Tipperary Brigade of the Irish Republican Army during the Irish War of Independence.
It was his actions that initiated the conflict in 1919.
He was killed in October 1920, on Talbot Street in Dublin, in a shootout with British troops and spies during an aborted British Secret Service surveillance operation.
Although sometimes written as Tracey, as inscribed on the commemorative plaque in Talbot Street, or even as Tracy, his surname is more usually spelled as 'Treacy'.