Go to the home page, click on Travel Questions, scroll down to the baggage section and click on "What is my checked baggage allowance?"
It states, "The Checked Baggage allowance is 20kg per person (15kg per person for all passengers travelling from the 1st November 2006 onwards). "
I use Ryanair frequently but I really wish they'd make up they're minds as to what the baggage allowance is. I suspect the temporary increase in the allowance from 15kg to 20kg was a bit of a "sweetner" when they introduced the baggage charge.