You can book online with checked baggage (and pay the relevant fees when doing so) but you can't check in online with checked baggage.
The check-in fee is �4 (per person, per flight) plus �9.50 for one bag (or �28.50 for two bags, or �47.50 for three bags). The maximum total weight may not exceed 15kg. (Otherwise you'll be charged �14 per excess kg when you check in).
You might well have travelled by now but, in case you haven't, I'll post again to state that my answer (above) was correct at the time I posted it but Ryanair now allows passengers with checked baggage to use online check-in. (You have to take your checked bags to a bag drop desk when you arrive at the airport but there are no actual check-in formalities).