DP says it all really.
If you cross up north from Scotland to Larne then it will be Scottish FIR (Hi Fishbangwallop) on 119.875 and handed over to Belfast City on 130.85 when coasting out. You will be passed on to Aldrogrove on 128.5 when West of Belfast city. As you go further west you may be handed over to Eglington on 123.625 or Shannon on 127.5..... however Shannon will be a problem this far north. General rule of thumb will be a handover to Donegal on 129.80 when crossing the FIR, you will be west of Letterkenny before you can "work" them.
If you cross from Wales to Eire then it will be London to Shannon direct.
Donegal airport is a wonderful little airport, not busy, extremely friendly bunch there and stunning scenery. You will need an Irish GAR form and Customs will be notified when you file it, again it is laid back and you wont be inconvenienced at all.
The GAR forms and other forms are available from my airfields site at
http://www.RuskeyAirfield.com , the Irish GAR also has the Fuel tax drawback attached.
If you require any more information then drop me an e-mail from the site, happy to help if I can, and Ireland is a wonderful country to fly in, mostly class G with quite a laid back attitude to flying. I live in Donegal, but at the other side of the county near the Northern Ireland border.
Good luck with your trip.
Jon