By definition Great Britain does not intend to encompass Northern Ireland, so GBR wouldn't have anything to do with Northern Ireland rail.That is so happy clappy, it's not true.
GBR so far is a whim, not a plan. It's just a case of a not very good rebranding rather than an attempt to build a functioning rail system.
Honestly, who cares if a crap system is forced to use a particular font or a brand? They might actually prefer to hide their **** behind an 'England and bits of Wales but definitely not Scotland or Northern Ireland pretending it's British' idenity.
As for definitely not Scotland, I'm sure Glasgow Central and Edinburgh Waverley are both there? Not to mention the permanent way itself, and all associated infra, level-crossings, signal boxes, bridges, etc., which are all owned and maintained by (and signed / branded) by what will become GBR.
It may work out, it may well not. But it's wrong to say there's not a plan or that it will be crap. Nobody can know that yet.

