You can take a ferry from Larne near Belfast to Stranraer in Scotland also or a P & O ferry Larne to Cairnryan/Troon however if you haven't your own car this might not be the best option. Aer Lingus also fly there. Aer Arann appear now to only fly into Aberdeen from Dublin & have cut their Inverness route since Ryanair muscled in.
e.g this site; http://www.ferrysavers.co.uk/larne.htm or http://www.ferrycheap.com/routes/Larne/Troon/POI
Easyjet do not fly fom southern Ireland since Ryanair would trottle them if they tried (who said flying isn't cutthroat). They do fly from Belfast to Edinburgh. Your best bets re: airlines are Aer Lingus & Ryanair. over 6 years ago