As for the Ryanair saga, my source says City management always said although Ryanair put large numbers of punters through the doors, it wasn't a proportionally high part of the profits. Not sure whether Mr A. or Belfast Tele is putting some spin on things
Given that all of Ryanair's routes were new routes from the airport (and STN was being vacated by AirBerlin) I would imagine that the city airport gave Ryanair a very attractive deal for 3 years on landing charges. Hence the reason why Ryanair didnt go after existing routes from the airport e.g. in competition with other carriers directly, as these most likely would not have been incentivised. However, while its probably true that FR ops weren't a big proportion of BHD profits, the volume of passengers probably assisted well in supporting the other businesses around the airport, the shops the cafes etc etc.
Given recent profits or lack thereof, I would imagine the City airport would be glad to have them back and enjoy whatever marginal return they could get from them.
I would say that Ryanair's presence added little to the bottom line of the airport, and given that each route was a 'new' route from the airport (well given that AB was pulling STN it probably was a case for an incentivised rate)