The relationship of the church and Spain is interesting. The Vatican supported Franco. Many Catholics in Ireland did not.
Of course, Ireland and Spain did have an interesting history. They were friends for centuries, and there was a healthy (illegal) trading history between the two countries. Allegedly there was a history of support of the IRA by the Spanish much later. Allegedly weapons and ammunition.
Here is a story about the Irish support of the Spanish against Franco. Listen to the song and to the names of those Irish who died.
https://www.youtube.com/watch?v=iq8_G7-9vKQ
“Tommy Woods aged 17 died in Cordoba.” That line in the song stops me every time I hear it.
The book that had the biggest influence on me was "The Cypresses Believe in God" by Jose Maria Gironella. It's actually a trilogy with the first volume covering the run-up to the Spanish Civil War as seen through the eyes of a Catalan family who are pulled and split up by complex competing loyalties.
The 2nd and 3rd volumes are about the war years themselves, and the period immediately after. But the first volume is definitely the best.
What got me was the mind-boggling complexity of Spanish politics, pulling in the opposite directions against the unity of the Spanish family.
I think it succeeds in not being an ideology-driven propagandistic narrative, but a multi-angle look at the incredible bloody tragedy which engulfed these noble people, both sides, all sides.