Paddy - like most Irish children at the time - experiences a distinctly Catholic upbringing and education. He attends Mass and receives Communion.
Ireland in the late 1960s was a Catholic society where even the laws of the state were influenced by Catholicism.
Contraception was illegal in Ireland when the novel was set. Perhaps this explains the Clarkes’ burgeoning family despite their failing relationship.
And although Paddy’s parents’ marriage breaks down by the end of the novel, they would not have been legally allowed to get divorced until 1996 - 28 years after the novel is set!
Paddy sometimes refers to other characters in terms of their religion. When he says, "He was a Protestant, a proddy" it shows us that he - and people at that time and place - classified and stereotyped others depending on their religion.
Paddy believes in God and prays to Him. But it is possible that - like many children and perhaps adults too - this is born out of a sense of tradition and duty rather than real faith.