Seamus Costello (Irish: Séamus Mac Coisdealbha, 1939 – 5 October 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.