As of November, the allowance will decrease again to 15 kg...
http://www.flyertalk.com/forum/showthread.php?t=578258
At some stations, check-in-agents are obliged to generate 1 Euro/pax in average for overweight luggage.
Ryanair deducts this amount in advance from the payment to the handling company, and the handling agents do their very best to recollect the money from the passengers.