Slight pet peeve: Northern Ireland dialects of English are not "British English"; they're Hiberno-English dialects. Northern Ireland is not part of Great Britain, nor is it British.
Folks in GB can generally distinguish someone with a Northern Irish accent from someone with a more southerly Irish accent.
e.g. Belfast, or Derry accent vs Dublin accent.
Not surprisingly so since Donegal, Cavan and Monaghan were also extensively settled by Scots immigrants, but were cut off from what's now casually called Ulster in 1922. So they had 300+ years of shared language versus 100 of separation.