i'm just back from a stag at the weekend in derry (which by complete coincidence coincided with their jazz and big band festival) butfirst thing when i check my phone after the whole weekend, this article pops up
Ireland has ‘undermined Britain for over 100 years’
one guy on it went to an atm machine in derry and it gave him euros as the euro and pound one were side by side. some places accepted euros too.
back in republic on way home, we stopped in a place for a drink in monaghan, they accepted pounds.
though northern notes, i would say you would have difficulty them being accepted in england. every bank in the north seems to issue their one, i had throughout the weekend first trust, bank of ireland, ulster bank, danske bank, northern bank (thought they got rid of these) and bank of england notes in change from places.
so yes, that border is very hazy for currency.
Everywhere in the UK takes Northern notes these days, usen't to be the case for a long time though.