The religion doesn’t mean much itself. It’s mostly now just a symbol for if someone identifies as Irish or British.
IRA were mostly atheist commies.
A lot of the Protestants in Northern Ireland are descended from settlers from Scotland in the 1600s. By the 1700s many of them were unhappy with how they were treated by England and emigrated in large numbers to North America (where they were often called “Scotch-Irish” or “Scots-Irish”). During the American Revolution they were among the most zealous supporters of American independence. Andrew Jackson is an example (he was born in America to immigrant parents).