I read this but have the feeling there is not an easy answer to that question.
https://www.cnn.com/2013/10/30/world/europe/northern-ireland-fast-facts/index.html
OK - Northern Ireland is not the same country as Ireland (while both countries are on the island of Ireland).
In Northern Ireland a majority of the population identify as British. The rest of the population of Northern Ireland Identify as Irish. All those living in the country of Northern Ireland are - in terms of nationality - British subjects. The identity divide is generally along religious lines, the catholics identifying as Irish and wanting to unite with the rest of Ireland (Republicans) and the protestants (Loyalists) wishing to remain British.
It all goes back many hundreds of years and was the cause of the sectarian violence seen up until the relative recent peace in Northern Ireland since the Good Friday Agreement was signed in 1998, which involved the paramilitary organisations on both sides ceasing decades of armed hostilities.
With both Ireland and Northern Ireland being in the EU economic zone there has been (effectively) no border between the two countries lately. With the UK wanting to leave the EU, the border may have to return - which the majority of ALL the Northern Irish population do not want to see - as it will possibly reignite the sectarian conflict and see the return of the paramilitaries (and their violence) on both sides in Northern Ireland.
That is as succinct as I can be. Even a lot of people in the UK do not fully understand it, to be frank. Beyond the UK very few people have even the tiniest idea.
I hope it helps explain the vagaries of the map of who identifies as what within this part of the UK.
I had a drunk Irish person complaining to me about yeah but what did Oliver Cromwell do!
FS get in the moment.
That's why there is violence people can never let go. Even 300 years later.