David,
Perhaps the Irish register is cheaper,not questioning that at all. However Ryanair is not an "originally" Irish airline, it is as far as I am aware a 100% Irish airline, with all A/C on the Irish register and it's headquarters at Dublin airport.
With relation to EUJet, there headquarters are at DebisAirFinance house at Shannon Airport, however I am aware of the Manston Airport involvement, not the details, so I would not presume to make an assumption on that on.
Can anyone else share any definitive details?
Rgds
P206