The Spanish Civil War lasted from July 17, 1936 to April 1, 1939. While both sides in the Spanish Civil War attracted participants from Ireland, the majority sided with the Nationalist faction.

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.