I can't speak for Ryanair's fleet, but on all our 737 models (ranging from 400 to 800), we have 4 masks per PSU, which is also the standard at Boeing (for the 737) if I'm not mistaken.
I would suspect it's airline specific. Our (old-ish) 737-400's have 4 masks on the left side only and the A319's are the same but changing sides alternately. It may be different on the NG aircraft though. If, by some miracle from above, we ever get any of the new Boeings, I'll let you know...
