Ryanair fly to Pisa from Standsted, which is within driving distance of Sienna. Make sure you allow oodles of time to change plans at Standsted, as if Ryanair are late on the incoming flight from Ireland, it's still your fault of you are late for the outgoing flight. You might be able to pick up seats on charter flights direct to Pisa from Dublin if you shop around with a few travel agents.