Seamus Costello (Seamus Mac Coisdealbha) was a leader of Sinn Fein and the Irish Republican Army, and sided with the Officials in the split of 1969, before becoming a founding member 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 was shot dead on 5 October 1977, 41 years ago this week, the apparent victim of an internal INLA feud. A biography and a tribute by Bernadette McAliskey.