The Irish Brigade fought on the Nationalist side of Francisco Franco during the Spanish Civil War. The unit was formed wholly of Roman Catholics by the politician Eoin O'Duffy, who had previously organised the banned quasi-fascist Blueshirts and openly fascist Greenshirts in Ireland. Despite the declaration by the Irish government that participation in the war was unwelcome and ill-advised, 700 of O'Duffy's followers went to Spain. They saw their primary role in Spain as fighting for the Roman Catholic Church against the Red Terror of Spanish anticlericalists. They also saw many religious and historical parallels in the two nations, and hoped to prevent communism gaining ground in Spain.

The Army Comrades Association (ACA), later the National Guard, then Young Ireland and finally League of Youth, but best known by the nickname the Blueshirts, was a paramilitary organisation in the Irish Free State, founded as the Army Comrades Association in Dublin on 9 February 1932. The group provided physical protection for political groups such as Cumann na nGaedheal from intimidation and attacks by the IRA. Some former members went on to fight for the Nationalists in the Spanish Civil War after the group had been dissolved.