The vast majority of Ryanair's passengers take ONLY cabin baggage. (Over 70% of travellers do that. On the Stansted to Dublin route many flights have no hold baggage at all). So that means that a lot of passengers are trying to carry as much as possible in their cabin baggage. I've witnessed lots of occasions where Ryanair staff (or airport staff working on their behalf) have rigorously enforced the 'one bag only' rule, insisting that passengers cram things like handbags and airport purchases into their already-full cabin bag. I've also seen Ryanair staff checking the dimensions of cabin bags (by attempting to put them into the 'cradle' which defines the maximum measurements for cabin baggage). However I've NEVER seen anyone actually WEIGHING hand baggage, so you chances of getting away with a bit of additional weight there are quite good.
I would have expected Ryanair to interpret their rules for hold baggage based upon "per kg or part thereof" (so that, for example, 15.1kg would count as 16kg) but their website only refers to 'per kg'.
http://www.ryanair.co...estions/table-of-fees
So you might well be safe up to 15.4kg (because that rounds to 15kg) but anything above 15.5kg would seem to be potentially expensive.
Chris