Hello Shawn,
If you’re travelling to Northern Ireland from a country outside the Common Travel Area via the Republic of Ireland, and wish to travel into Northern Ireland for a day trip or short stay, you must follow any requirements that have been introduced for that country, including post arrival testing on day two for non vaccinated travelers.
It is also said that you should take a rapid lateral flow device test (LFD) and only travel if your test result is negative before you begin your journey to Northern Ireland
You can find more detailed information on
this pageBest wishes,
Antoine