It is important the public check the Foreign Office Travel advice website and determine what is necessary for entry to that country by UK citizens.
Please check regulations in each jurisdiction before you plan your trip as requirements are subject to change at short notice. https://www.gov.uk/foreign-travel-advice or https://www.dfa.ie/travel/travel-advice
That includes finding out whether PCR tests are required or if a period of self-isolation is necessary on arrival on return to NI.
