The first question I ask in research is “Were they Catholic or Protestant?” This is important because of the different research approaches to each. If you don’t know, then chances are they were Protestant. Catholicism is a culture as well as a religion. So something usually passes down in some form. Protestantism is different. Loyalty is not necessarily to a particular denomination, but more individual. In other words, one could switch denominations, often as simple as transferring membership. Salvation isn’t based in church or ritual, but in the personal faith. Catholics held church and ritual to be very important, so they should appear in the registers. The question then is: How complete are those registers?
In Irish research, you will find Protestants documented more in the leases, marriage bonds and wills. However, saying this, most Protestants were still poor tenant farmers, just like their Catholic neighbors. Catholics of the upper class did appear in these records, but there were often legal restrictions, for example on how long they could hold a lease.
If you have spent much time in Irish church registers of all denominations, you already know there was conversion both ways. Mixed marriages were more common than you would think. In another twist, beginning in the 1840s, it was mainly Protestants who converted to Mormonism. A religion that was neither Catholic nor Protestant. That began to change after the partition of the country in 1921; then it became Protestants in the north and Catholics in the south.
Just be aware that knowing the religion of your ancestors is important. Whether they practiced may even be beside the point. The emphasis is on what records would document that particular culture at that particular point in time.