The highly informative CGP Grey retreated to the Forest of All Knowledge with his dog to learn why Northern Ireland doesn’t really have a flag of their own. Since 1972, however, the country unofficially adopted the Ulster Banner as representation in its place. This particular flag has a rich, traditional history, though it is also somewhat contradictory in its design.
So though the flag is composed of traditional Northern Irish symbols mostly, it’s not official because it never really was 100% and the government that used it doesn’t exist. And when the most divisive issue in your land is should it be part of a kingdom, having a crown on your flag isn’t exactly neutral and is also why there are periodic calls for a new Northern Irish flag.