Northern Ireland is on the island of Ireland but part of the United Kingdom Mexpiper. It's not the place here to go into the history of how it came about but just a couple of general observations...

The flag isn't just Northern Ireland but the flag of the Province of Ulster. The historical Province has nine counties but six of them are in the UK and three of them are in the Irish Republic. Although the flag is used for the Six Counties it actually represents nine counties.

Although England isn't Celtic as such there are fringes - they have probably assumed (as most people do) that Cornwall is in England for example. There are also places in England that used to be in Scotland, such as Berwick upon Tweed, and places in Wales that used to be in England such as Monmouthshire. Also England isn't a complete desert in terms of things Celtic, there are some English tartans even and quite a few pipe and drum bands whose members may not have a Celtic ancestry.