If [or rather] when fuel prices increase, and thus air-fares increase accordingly, the pax wanting to travel at the new higher fares will be business travellers or more affluent leisure travelers. I would venture that neither of these two groups would put Ryanair as their carrier of choice.
To be fair, I think this is an issue that is facing all lo-co carriers: in an era where cheap fares are no longer possible, how do you differentiate yourself from the competition? The obvious answer is customer service, and I would suggest that service is not Ryanair's strongest selling point.