As dispicable and thoroughly unpleasant a man he is, O'Leary is not serious in his suggestion. I would bet a large sum that if the regulator called his bluff and said, "Ok Mr O'Leary, you can do away with seatbelts and have standing room only," he 'd never really do it. Imagine the carnage after just a few days. Pax crushed as the airplane brakes on the runway, people bouncing off the ceiling........
And even if he was mad enough to do, and pax where mad enough to go for it, I think I'd just about have enough faith in the professionalism of Ryanair crew to refuse it.