Both Ryanair and Jet2 fly to Girona from Belfast. EZY offer BCN from Belfast. Plenty of options to Girona or Barcelona from Dublin.
Depending on where you are heading on the Costa Brava, Barcelona airport may not be the most convenient choice. It's on the wrong side of the city. Eg. last September it took me almost 3hrs to travel from BCN to NW of the Costa Brava. Last month we used Ryanair from Belfast to Girona and it took us 50mins from leaving the aircraft to arriving at our destination. That was disembark, passport control, collect car and the drive. Thats before we even get into discussing the drive from belfast to Dublin!