Most Irish in the Spanish Civil War fought on the Loyalist (leftist) side.
Not sure about that; its only Wikipedia, but:
“Encouraged by the Church hierarchy, Eoin O’Duffy, leader of the fascist NCP, started to recruit a brigade of Irish volunteers to fight in Spain in defence of the church. By late 1936 some 7,000 men had volunteered, of whom about 700 were selected, and in November 1936 these sailed to Spain, where they became the XV Bandera (battalion) of the Spanish Foreign Legion, or “Irish Brigade”...In all 320 Irish men served with the International Brigades, a quarter of whom were killed in action.”
As a Catholic culture there was little to attract Irishmen to the communist side; the well-publicized atrocities against the Church in 1936 attracted a lot of support among the Irish for the Nationalists.