I, too, think that Ryanair sometimes opts to pay, for example, the £10 UK Air Passenger Duty rather than pass it on to the customer. What this means, obviously, is that when one flies from Stansted to Belfast and back, say, for £2.02 (including the debit card fee) or from Stansted to Dublin and return for nothing in effect Ryanair is subsidising you to travel as, presumably, it still has to pay the APD in addition to a PSC at each end.