I think I agree with most of you when I say religion is not the main point in this conflict. But it also isn't a matter of political parties or social differences. In my opinion, mainly the ethnical differences are the crucial reason for this conflict. There are living two nationalities in Northern Ireland (well, at this point we could discuss if the “british” nationality really is a nationality), which don't have a good history concerning the relations between them. Especially the british don't really have a clean record. That they have different religions is just a fact, which the leaders of the conflict parties can use for their propaganda to influence their followers. Also that the two groups have an different political view (republicans <->royalists), is not important. To say it the other way around: Even if both parties would be for example catholics, and also Ireland would be a kingdom like Great Britain, the conflict would still exist.
To come back to Jonas' question: Cal and his "friends" aren't very religious, even if they say it. With the identity “catholic”, they just can define themselves more from the “brits”. But, like others before mentioned, to be religious doesn't prevent from using violence, often the violence is even legitimated by religion.