They definitely don't as said above the pressurisation system is automatic and does its thing. There maybe time when technical issue requires the crew to go "manual" but its not common.
It more likely to be the number of cycles the aircraft do and the amount of pax and the number of cleaning cycles. Although to be honest I can't see that being that much different to any other loco.
What it might be is the flight profiles are shall we say more aggressive ie they climb rapidly and they decend rapidly the auto systems may have two or more depressurisation schedules depending on the rate of decent ie decend at 2000ft/min and you get 500ft/min 3000ft/min plus and its 750 per/min (I have no clue real numbers for that type)
Also what sometimes happens with high rates of continuous decent is the planes catches up with the cabin ie the cabin becomes less pressurised than the outside air. When this happens a safety valve opens because the plane is designed to be under pressure on the inside and negative pressure would result in the tube collapsing. When this happens the cabin will descend at the rate the aircraft is descending which may be over 1000ft/min which does cause discomfort.
The next thing it could be.. you can read the huge toxic cabin air thread to find out about leaking oil seals and what that's all about. or google aircraft organophosphate poisoning.
I am 99% sure its nothing to do with the cabin pressure per say.
As I personally never fly with Ryanair I really can't give comment from personal experience.