People are not going to bother driving down to Dublin for a London flight, and probably not for the bucket and spade trip to the Canaries or wherever, so they are safe. Long haul is hard to justify perhaps. I assume Air Transat, Air Canada, Delta, US Air, AA, United, Emirates, Etihad and Turkish all look at where their customers ex DUB are coming from (they do have addresses from bookings after all) and decide whether to launch an extra service out of BFS,ORK,SNN or wherever based on that. If there is a particularly large portion of those pax coming from Ulster for example, it doesn't necessarily mean they would launch a new service to that part of the Island, as instead of having one profitable route they may end up with 2 unprofitable ones after losing those pax from the original service. Then tie in things like cargo demands, pre-clearance for the US and things get complicated.
Both airports in Belfast are making money, sure there aren't the shiny widebodies for now, but people's jobs at those airports are safe, that's something to be very thankful for indeed