It's often said that nationalists are predominantly Catholic and unionists are predominantly Protestant. Indeed sometimes they become interchangeable and Protestant/Catholic are used to mean unionist/nationalist. Sometimes they are even combined into their own labels, such as the acronyms PUL (Protestant Unionist Loyalist) and CNR (Catholic Nationalist Republican).
I think the use of these terms, whilst obviously excluding "opposing" religions from holding the "opposite" view; Catholic Unionist and Protestant Nationalist, I think in a Northern Ireland that is becoming more secular and even more irreligious, these terms can be misleading and assume that a political allegiance is inevitably tied to a religion.
Since we have seen a gradual trend towards non-religion in Northern Ireland, and more people of both religions holding both political allegiances, I think the political terms Unionists and Nationalists should simply be used on its own when describing the communities, not because I am offended but because I think it would be more applicable. NR and UL maybe?
Out of interest as an agnostic, how many people on this forum would describe themselves as truly Protestant/Catholic/other religion and how many are religiously speaking non-religious?