Michael O'Riordan was the founder of the Communist Party of Ireland (3rd) and also fought with the Connolly Column in the International Brigades during the Spanish Civil War.

The Connolly Column was the name given to a group of Irish republican socialist volunteers who fought for the Second Spanish Republic in the International Brigades during the Spanish Civil War. They were named after James Connolly, the executed leader of the Irish Citizen Army. They were a company-strength unit of the XV International Brigade, which also included the US, British and Latin American battalions in Spain. The name is now retroactively applied to all Irish volunteers who fought for the Spanish Republic.