Agreed, Knock-Ireland West will probably be a FR base before Derry but you got to remember that Ryanair have a lot of aircraft arriving over the next few years. Once they have a few routes established out of a particular airport and they get a reasonable airport deal, its likely they will base an aircraft here. Ryanair have over 100 new aircraft being delivered over the next few years that will need to be based somewhere. I hope that Aer Arann will also base an aircraft in Derry and develop more routes from the northwest as there is a lot of potential here.
By the way, Knock is really the main airport in Connaght as Galway and Sligo airports are quite small so they can attract passengers from all over the province.