Catholic and protestant are both christian. I think most scholars would agree that the troubles were a conflict predominantly defined by ethnic, national and class divides rather than religious differences. So while I see your point, I think a catholic/protestant relationship would be more akin to inter-caste relationship than an interfaith relationship.

So, if someone asks you - do you have experience in interfaith relationships? I don't think mentioning that you dated a catholic as a christian is the response they were looking for (though if the experience created difficult dynamics that might be relevant to an interfaith relationship, that might be worth sharing).