Or rather a MAG-wide deal?
Possible but unlikely.
The management of each airport is allowed to operate independantly of each other in virtually every commercial aspect and individual Airport Directors are expected to justify their profit or loss accordingly.
Indeed the airports compete directly with one another (an example is the perfectly legit' commerical deal Manchester have with train operators, which gives passengers from the North and the South Bank of the Humber willing to fly from Manchester, a very good deal on direct train services which operate direct to Manchester Airport for a very cheap fare. The trains are regular and efficient and also operate H24).
Its extremely unlikely that Ryanair would accept the same terms for operating into each airport, they'll be looking for the deal that suits them for each individual operation.