Ah, of course. +1 Saor Alba! That is an important fact. I should have said that I thought that they are the only 'Accepted' or 'Authorized' banknotes in NI. MauMauNotes brings up a great question, I think. I would think that in Ireland, these notes dominate, and like most places with their own currency, they are preferred for use when possible, even if the other notes are allowed by law or by custom.
As I wondered about the crown dependencies, possesions, etc with their own curency, I went to Wikipedia and found a plethora of information. The section on everyday ise and acceptance perhaps best answers Mau Mau Notes' question. Link