Hi Adam,
Not sure who you should be talking to on the way over but when you get into NI Aldergrove App are very helpful and should be able to give you a basic service to the Shannon FIR. From there the route I take as I find is the most scenic is via Killybegs on the South west of Donegal county then northwards to EIDL. Shannon are generally helpful but coverage around Donegal is marginal at best under 3000. If you don't manage to make contact with them make sure you advise Donegal on contact and ask that they call Shannon to advise. I would be wary of coming in from the north east as there are some high mountains up to about 1800 - 2000' so this is an area you want to be familiar with or have perfect VMC.
Donegal itself is great wee airport with a nice cafe for a coffee in the terminal. The ATCO's are very helpful and there is a beach which runs parallel to the runway which makes for a nice walk in the afternoon. Airport facilities are top notch and AVGAS is available. Bit of dip in the runway though!
I intend on using the landing voucher before the month is out also, but you may also be interested to know that touch and goes are free so you may do one or two when coming in or going out so you can really appreciate the scenery. Worth a pop over to sligo when your in the area another great spot. Also, Enniskillen is only 30 minutes away and with a beautiful Maule M7 on amphibious floats you could end of the trip with half hour on the lakes!
If you need anymore info feel free to PM me.
Cheers
Ryan
EDIT: There is a form Donegal request you to fill in for arriving UK flights (which I think is on their website if you take a look, if not I can get a copy for you) and of course a standard flight plan as crossing the FIR.