I think this often-stated notion of '7m Belfast passengers pa so we should have xy or z international services' overlooks some important unique characteristics of the NI market.
First, most of the services that make up the 7m are flights to GB - someone will be able to provide the actual figure but this is the bread and butter business of airports in NI.
Secondly, since NI has obvious political and cultural links to GB but is physically detached, the number is distorted because the same kind of journeys if they were in GB would be made by train or car.
Thirdly, the population of NI is tiny, about the same as Kent. It's profile (for all the wrong reasons) allows it to punch above its weight already.
Fourth, Dublin and the new improved motorway that gets you there from Belfast in less time than it takes to drive from Central London to Gatwick.
The fact that Belfast has only just been able to attract KLM Cityhopper on a modest frequency should serve as a reality check.