I really dont think that Northern Ireland needs any more flights to any of these places, other cities area available!!! MAD,CPH,VNO,SVO just as a wild selection. Why on earth to all airlines that use NI airports like lambs to the slaughter keep all going to the same place only for a price war to follow and they all pull out again!!!
Given the dearth of private enterprise and non government supported commercial activity, NI is limited to routes were the business model doesn't need a healthy number paying top dollar up front to make the numbers add up. Hence sun routes for holidays and UK domestic for friends and relatives. If someone from the continent is coming to (the island of ) Ireland, chances are they'll pop up in Dublin and make their way North.
Belfast does have a growing inbound tourist market but there's reasons why it's way behind the rest....