The north coast of Spain, fronting onto the Bay of Biscay is nearest to Northern Ireland but, compared to more southerly parts of the country, it has few tourist resorts. It also has no airports served by flights from Northern Ireland.
There are no flights from Belfast City (George Best) Airport to anywhere in Spain:
http://www.belfastcityairport.com/template1.as p?pid=39
From Belfast International, the shortest routes to Spain will be the scheduled flights to Barcelona and the charter flights to Girona (just north of Barcelona) and Reus (just south of Barcelona):
http://www.belfastairport.com/en/content.asp?a rea=2&id=210
As well as serving Barcelona, those airports serve the Costa Brava (Lloret de Mar, Tossa de Mar, Estartit, etc) and the Costa Dorada (Sitges, etc).
easyJet's scheduled service to Barcelona (from Belfast International) doesn't start until 3rd November but it provides a guide to flight durations. Their service is scheduled to take 2 hours 40 minutes, in either direction. Airline schedules are based on the times from 'push back' to 'arrival gate'. The actual flying time is usually 20 to 30 minutes less than the scheduled flight duration.
Chris