... to keep your hand luggage with you in the cabin
This is a new rule from their FAQs:
"Due to cabin space limitations only 90 cabin bags (55 x 40 x 20 cms) can be carried in the cabin, any remainder will be carried free of charge in the aircraft hold."
It seems pretty clear that there is no option, although I would imagine that if there are 91 people on the flight and there is room they won't force the last person to board to gate check their bag.
Given that Ryanair 737s carry up to 189 people, this rule means that more than half on a full flight will need to gate check their bag.Edited: 7:04 am, December 30, 2013