An update:
Of course they rejected my claim. The important parts:
We sincerely regret the delay to your flight FR583 from Pisa to London Stansted on the 12/05/2013, which was due to an unexpected safety/technical problem with the aircraft due to operate your flight.
It was necessary to delay the departure of this flight until the fault was rectified and the aircraft was cleared for operation by our Engineering Department.
Ryanair is committed to provide on time services for all passengers and is Europe's No. 1 on time airline.
Occasionally, there are situations which are outside our control such as technical faults that disrupts our flights.
..... Blah blah, provided refreshments, etc.
As this delay was outside Ryanair's control (Extraordinary Circumstances), we regret to advise that no compensation is due under EU261.
So next steps will be to write back citing the cases where it was ruled that technical problems are not 'Extraordinary Circumstances'. After that I'm not sure. Dealing with the Italian authority seems to be a lost cause from what I've heard. Maybe it's then time to go with a no-win, no-fee agency and see what they can do.