I think the Air conditioning is powered by the APU (gas turbine in the tail of the aircraft) on the ground or Fixed Electric Ground Power. Inoperative APU is no

I think the Air conditioning is powered by the APU (gas turbine in the tail of the aircraft) on the ground or Fixed Electric Ground Power. Inoperative APU is not safety critical so airlines are allowed to fly with this defect. I'm guessing you weren't connected to ground power because Ryanair save money by only buying the absolute minimal services from the Airport; the FEGP is usually part of the air bridge structure in modern terminals but Ryanair flights seem to mostly board up stairs even when an air bridge is available.