I’m not a historian
and this blog is pretty much inactive, but I’ll try.
An extremely simplified version would be that Ireland, being under English rule in the late 16th century, was forcefully colonized by the English (and Scottish). Since both England and Scotland were by that time protestant while Ireland stayed catholic, the newly implanted families stayed different from the Irish not only by the nationality, but by their religion as well.
I don’t want to go into details about the whole separation as it’s a very sad and bloody story, but these differences ultimately caused that in 1922, the Northern part of Ireland opted to stay with Great Britain, while the rest of it became independent.