I'm from there myself & it's a complicated history.
I'm simplify it a lot here but....
The 6 counties of the north were taken by force by England, the English found the Irish natives difficult to control. So this lead to the plantation of Ulster, naming taking thousands of families - mostly from Scotland and moving them over to Northern Ireland. Removing Irish Catholics from their homes/business were they were given to the planters. It was organised colonization.
The planters are the Protestants who were told to stay loyal to the Crown - this has been engraved throughout generations. The Catholics are the displaced Irish.
Irish Catholics were still the majority in a lot of counties at the time, so the British decided to gerrymander the voting system. Instead of one man one vote, you only got a vote if you owned property of businesses. Thus the minority protestants had control over local government, deciding on housing/jobs, most unwilling employ catholics. This was one of the reason for the march on Bloody Sunday.
The IRA were initially born as a result of the treatment of Irish Catholics...