The conflict in Northern Ireland is essentially cultural, not religious. It is an ethnic dispute between two distinct groups, similar to the conflicts seen in the break up of the former Yugoslavia.
One group has a British background, largely descendants of Scottish settlers planted in Ireland several hundred years ago. They and their ancestors are Irish by birth, but they defend their British ancestry for cultural reasons, and also for the sake of self protection - since they are surrounded by a majority of a different background. Historically most of them have a non-Roman Catholic Christian background.
The other ethnic group is largely native Irish, with cultural roots in Ireland. Many of their ancestors lost the land that the settlers were given. Historically, most of them have a Roman Catholic Christian background. In the past, being Roman Catholic in the United Kingdom meant having a lower social status, and that has a major influence on their view of history.
The two groups use “Catholic” and “Protestant” as a shorthand for the cultural background that they come from, but the dispute betwen them is not about religion itself. Their religion is a characteristic of their ethnic background, not the cause of the conflict.
You see the same type of conflict in the Balkans or the Caucasus. Warring groups there have different religions (such as Muslim or Christian), but “religious” disputes are not the source of conflict, but a symptom of an underlying ethnic dispute.
The two Northern Ireland tribes each have a distorted mythology of history. Most people are low-key about it, but each group contains highly irrational people who do not accept any deviation from their holy view of history. The danger for an outsider is that you will be persuaded by the force of their blazing belief, and will not understand that you are only hearing a very limited viewpoint.
In particular, be aware that most “historical facts” that people honestly believe are likely to be untrue. They are based on the propaganda of the past, and have little basis in real fact.
Of course that applies to every country in the world, including the USA. Luckily, most of these myths don’t kill many people.