SD, from Ryanair's FAQs:
"If you do not travel on your booked flight the air fare, fees and charges are non-refundable but you may apply in writing within one month of the date of travel for a refund of the Government Tax paid. Government tax refunds are subject to an administration fee of £17/€20. If the refund amount is less than the applicable administration fee, no refund will be made."
Mr O'Leary is so generous, I'm sure that the Administration charge is more than the amount of duty paid, but at the same time no mention is made as to whether the charge is per passenger or relates to multiple passengers, which would make it worthwhile if claiming for more than one flight.
Good luck trying!