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.

Many Irish Independent newspaper editorials endorsed the idea, and on 10 August 1936 it published a letter from O'Duffy seeking assistance for his "anti-Red Crusade".

Cardinal McRory stated 'There is no room any longer for any doubts as to the issue at stake in the Spanish conflict...It is a question of whether Spain will remain as she has been so long, a Christian and Catholic land, or a Bolshevist and anti-God one.

[9] One author has described some of the Brigade as 'social misfits who saw themselves as the twentieth century's Wild Geese, and rural lads talked into enlistment by rhetoric from the pulpit.

Large crowds gathered to sing 'Faith of Our Fathers' as volunteers were blessed by priests and handed Sacred Heart badges, miraculous medals and prayer books.

From their training base at Cáceres the volunteers were attached to the Spanish Foreign Legion as its "XV Bandera" (roughly, "fifteenth battalion"), divided in four companies.

There was also dissension between the volunteers and their Chaplain, Father Mulrean, who made himself unpopular due to his public berating of the Irishmen for their 'immoral' behaviour in front of Spanish officers.

[17] On 19 February 1937, the Irish Brigade was deployed to the Jarama battle area, as part of the right flank at Ciempozuelos, but when approaching the front line they were fired upon by a newly formed and allied Falangist unit from the Canary Islands.

In February, the prospect of more Irish reinforcements arriving was precluded by the de Valera government passing a law prohibiting any more volunteers to leave for Spain to fight for either side.

[27] One Brigade member noted in his diary that morale was collapsing, 'Bandera cracking up – all men are getting sick & weak, dozens going to hospital each day...great rumours about going home.

Showing their displeasure with the affair, the volunteers from County Kerry and from the north of Ireland marched off separately from the main contingent, disassociating themselves from O'Duffy.

Fearghal McGarry writes, 'As it dragged on and atrocities such as the German bombing of Guernica, a village in the Catholic Basque region, became known the idea of Franco leading a religious crusade became more difficult to sustain.

Even the Catholic church toned down its pro-Franco stance when it was revealed that the Christian Front had gained control of its national collection for Spain under rather dubious circumstances'.

[33] Historian Fearghal McGarry writes that 'they left behind them fifteen dead compatriots [and] six hospitalized legionnaires[34] The Irish government destroyed its files relating to the Brigade in May 1940.