Confused about the relgious difference between Dada and his daughters

I get that Dada was Irish (Northern Irish, right?) and an Orange Man. But the girls were practicing Catholics, with Iris especially devout & considering becoming a nun. I assume the Mother was Catholic too? Was she Irish or English? And as inter-faith marriage was taboo then, I'm wondering how this occurred. Was there a back-story that I missed? Thanks in advance to anyone able to help me understand this. Loved the series!