Seat belts are generally pretty much the same on all airlines, however what happens often is that during maintenance when the ends are frayed they just cut off and re-stitch the end of the belt, so it becomes shorter and shorter until it is unusable then it is replaced. Especially at Ryanair I'd imagine this to be common, so try get on early and see if surrounding seats have longer belts if yours is too short and then ask to switch.