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.

Eoin O'Duffy was an Irish revolutionary, soldier, police commissioner and politician. O'Duffy was the leader of the Monaghan Brigade of the Irish Republican Army (IRA) and a prominent figure in the Ulster IRA during the Irish War of Independence. In this capacity, he became Chief of Staff of the IRA in 1922. He accepted the Anglo-Irish Treaty and as a general became Chief of Staff of the National Army in the Irish Civil War, on the pro-Treaty side.