Well until 1973, the Northern Island had its own democratically elected government … so it was like a state (of USA, Australia, a province of Canada, a country of the UK ) … .So there is a royal warrant assigning the Ulster Banner … but in 1973 it went back to being a territory under administration, so the entity the royal warrant gave the flag to no longer existed, it became a territory of the UK and so the union jack is the only known flag approved.
In 1998 they got autonomy back and the Northern Island Assembly has a flag, but its not the flag of Northern Ireland, nothing unique assigned yet.