The destinations map on the Belfast International Airport website shows Fuerteventura as currently served only by charter flights:
http://www.belfastairport.com/en/content.asp?a rea=2&id=210
Clicking through to list of summer charter flights shows the
only operator as Thomas Cook (with flights on Saturdays only):
http://book.flythomascook.com/cheap-flights/to -FuerteVentura-Spain/from-Belfast-UK/
No flights are shown from May onwards. However, it should be remembered that charter flights primarily exist to be used as part of a package holiday. It could simply be that Thomas Cook won't offer any flight-only deals on that route until nearer the departure dates, when they'll know how many spare seats are left after they've sold as many package holidays as they can. To check with Thomas Cook, phone 0871 230 2406 (10p per minute, Mon-Fri 0900-1900).
There are no flights to Fuerteventura shown on the destinations map for Belfast City Airport:
http://www.belfastcityairport.com/Airport-Info rmation/Available-Routes.aspx
Chris