Certainly in the USA, if you jumpseat while on duty, then that counts as regualrr flight time, and it has caused problems in the past.
FWIW at my company (UAL) deadheading issues form a significant part of the contract. Basically if there are absolutely no seats available then a gate agent could ASK a crew member to jumpseat, but the crew desk has to be consulted first, for the flight time limitations listed above.
As others have said, there is more to this than appears.
Can someone quote the RyanAir contract here?