Seamus Costello (Irish: Séamus Mac Coisdealbha, 1939–1977) was a leader of Official Sinn Féin and the Official Irish Republican Army and latterly of the Irish Republican Socialist Party (IRSP) and the Irish National Liberation Army (INLA).
He argued for a combination of socialist politics on economic issues and traditional "physical force" Irish republicanism. He is best remembered for the founding of the IRSP and the INLA. He was a victim of the feud with his former comrades in the Official IRA.
Read more about Seamus Costello: Early Life: IRA Border Campaign, Political Activism, Founds INLA and IRSP, Death