I just visited NI and the Republic. NI is part of the UK so you need nothing to fly over or fly back. The countryside is just as beautiful as the South, in my opinion. Regarding the Republic (or Ireland as it is sometimes known) the situation is more complicated and yet simpler. If you look for Ireland on the UK's Red List, Amber List or Green List you will not find it. It isn't there. This is because it is in the CTA, the Common Travel Area. This gives it the same travel status as the Isle of Man, the Bailiwick of Guernsey, etc etc. So, if you're entering from and leaving to the UK you need precisely nothing. I can confirm this because I've just done it. As long as you don't stray outside the CTA you need nothing. I couldn't really believe it either, but it is 100% true. No passport (although you should definitely have one with you), no tests - before or after, and no Passenger Locator Form. Nothing. I repeat NOTHING.
How come? Ireland has been stuck in the CTA along with Guernsey etc because of the Good Friday Agreement. This is an "international" agreement that was reached under war criminal Tony Blair. It was vigorously supported by anti-British Democrats like Clinton and his pals. It makes numerous concessions to the Republicans, including vaguely worded unimpeded (ie. no checks) travel between the UK and Ireland. The GFA is why Brexit will never be genuinely implemented and why European Courts will hold sway in the UK, for as long as it exists. Just my view.
