Last time I looked the Giants Causeway was in the UK - no need for customs, SB only.
I think the closest airfield would be Causeway (their website seems to be down but some info at
Hangarage and Tie down space, Hangarage - Causeway Airfield, Clubs & training, advert ID=16745). This is just South of Coleraine.
If you need tarmac, then I it is a toss-up between Derry/Londonderry and Aldergrove. Ballykelly is not an option - unless you're a Ryanair 737 and a little confused.
