Onward connections to where via BCN ? The IAG Hub in Spain is MAD !
Vueling have a large short haul hub at Barcelona with flights to destinations across Europe and into Africa. Most of those cities might not connect well time-wise into BCN-BHD and/or are destinations for which a Belfast traveller would need a very significant detour to transfer in BCN - however key Spanish regional cities at least probably have good connections into BCN-BHD