Allan Conn: I have seen Ulster Bank notes being refused in Oban.

That's understandable, but probably a throwback. At one time English banknotes were at parity with Irish banknotes, so treated as equivalent. In 1979 changes in Ireland (republic of) meant that parity was lost, and Southern Irish banknotes had to be treated (in UK) as foreign currency with an exchange rate.
Working in the bank at that time meant identifying whether an 'Irish' banknote presented was Norther Irish (drawn on a bank in Belfast) and exchangeable 1 for 1 with UK currency, or Republic of Ireland (drawn in Dublin) cashable only by using the relevant exchange rate. This problem continued until 1999 when the Republic adopted the Euro

There was an earlier time, in the 70s, when most UK banks would not accept any Republic of Ireland banknotes although they were at parity because repeated bank strikes in the Republic meant that the UK banks could not get the value for them.