Just got off a Ryanair flight actually!
Maximum is 15kg per bag, and believe me if your even 0.6 of a kilo over, you will be charged.
Just had a nice 80 euro charge for access baggage to pay, but at the end of the day its still cheaper than BA or Monarch!
I think next time I will remember to check 2 bags in, rather than just the one and then be over weight on the return leg.