Today's Guardian reports that Ryanair is changing its cabin baggage policy (again) with effect from November: https://www.theguardian.com/business/2018/aug/23/ryanair-introduces-new-restrictions-on-cabin-luggage
If I read it correctly, passengers with priority boarding will still be able to carry two bags onboard. The number of priority passengers will be limited to 95 to ensure there is space in the lockers.
Passengers without priority boarding may carry a small underseat bag. All others must be checked, with an £8 fee for a bag of 10 kg or less.