Huh. Including England/Scotland/Wales but not Northern Ireland feels extremely arbitrary, to the point that I wouldn't be surprised if it ran afoul of some kind of discrimination law in the UK.
It's much simpler than that - Northern Ireland doesn't have an official flag.
In principle, Unicode 5.0+ emoji scheme allows encoding any subregion that has an ISO 3166-2 code, so just as Scotland is GBSCT(), Northern Ireland would be GBNIR(). But England, Scotland, and Wales are the only ones included in the "recommended for general interchange" list. I don't know why they haven't added any more subregions to that list, though.
