Well every passenger over the age of 12 that departs from Knock adds €10 to the kitty regardless of what airline they fly with. In 2008 no subsidies were needed that I am aware of and the forecasts are that the 08 record pax number (629k) can be exceeded this year. Hopefully if that is the case the airport won't need a subsidy.
As for the main operator, Knock isn't a Cork or a Shannon so they have got to grasp every route they can get. I suspect Ryanair get a great deal at Knock and as such are unlikely to up sticks any time soon, although this years expansive continental schedule as listed above may not be sustainable longer term.
The presence of BMIBaby, Aer Lingus and soon flybe at the airport would give you hope that if Ryanair were ever to leave, much of the capacity would be taken up by other airlines. I don't think there is a need to fear expansion, the fact that it is holding steady in the midst of an economic crisis when other airports numbers are crashing shows that there is a lot of potential at Knock.