I'd read the terms and conditions as a starter. I remember reading that if you had a cast you need to pay for two seats with Ryanair. Maybe you should book the special assistance, to get you to the gate on a golf cart. Not sure if that costs extra.
I travelled Ryanair Dublin London with a 13 year old in a cast and 1 crutch. The cast was below the knee. We had no problems or issues apart from the obvious ( the walk to the gate and climbing the stairs of the plane was very slow and tiring )
I've seen passengers with crutches take their seat and then give their crutches to the cabin crew who then store them. They were returned at the end of the flight.
If you are unsure then contact details for Ryanair are here. Aer Lingus is no problem.
Might also be worth checking what gates you will be using if your mobility is restricted on crutches. Having recently arrived back in Dublin after midnight with Aer Lingus with a young sleeply child, I then had to walk to from Finglas where it seems they parked the plane carrying said child because they don't deliver buggies to the aircraft at that hour. Never again!
My wife travelled with crutches last week on Aer Lingus. She had broken her ankle and had a boot and no cast. There is no issue with travel, the crutches are not considered as carry on. They fit in the overhead lockers without being reduced in size. When you get to the gate let the gate staff know you need a little more time to board the plane and you will be boarded first. You may want to check in advance that the plane is using an airbridge and not parked at one of the remote stands where all passengers will be required to board by steps. If climbing any distance is a problem then you need to book assistance when checking in.