It does tend to vary between different airports. (That's partly because the staff at some airports are actually employed by other companies, such as Servisair, even if they're wearing Ryanair uniform).
The staff at Stansted are quite strict. (I've seen a few people who seem to have got away with 'stretching the rules', but I've also seen plenty of people challenged about the weight or dimensions of their bag). On return flights, from overseas airports, I've seen staff who've carefully studied every bag at some airports, whereas there have been other airports where you could quite literally include the kitchen sink in your hand baggage and nobody would notice!
The key rule which is almost universally enforced, however, is the 'one bag only' rule. You must put everything (including handbags, laptops and post-security purchases) into a SINGLE bag (which conforms to the requirements for weight and dimensions). If you've got two bags you risk being denied boarding.
Chris