I'm so happy to hear that you enjoyed your road trip around the island, and that you got to see the house your father grew up in! :)
The Northern Irish Pound Sterling banknotes are indeed a valid means of payment in Northern Ireland and although it is legal tender in the rest of the United Kingdom, a shopkeeper can refuse to take them. I'm so sorry about the inconvenience. Have you reached out to your local bank or currency office yet? They will be able to trade them for normal pound notes.
Let me know if you have any other questions, and keep me posted!