“Ireland was populated by Irish people?”
The northern Irish people want to be part of the UK.
“The northern Irish people want to be part of the UK.”
No, the Scots and Welsh the British relocated to Ireland several hundred years ago want to be part of the UK. The Irish (which are almost a majority, if they are not already in Northern Ireland) want to be part of the Irish Republic.
Maybe about half do. The country used to be 2/3 Protestant and 1/3 Catholic. The Catholic population is now over forty percent, and most of them support unification with The Republic of Ireland. The upcoming decades, and demographics, will decide the issue.