I'm pretty sure you can bring a bag along, but in true Ryanair style, it'll cost ya!!
They'll charge you for the airport check in and, asfaik, will carge you the rate per kg for the bag (something like €8/kg).....I'm not 100% about this, it was just I considered the doing the same and this was what someone said to me, plus I had the added complication of having kids so apparently would have caused a hassle as they were on my ticket & they can't done online check-in on their own so I may have been required to do an airport check in for all of us.
If haven't booked your tickets, you could book them as separate flights....but the hassle sometimes isn't worth the saving. Mind you, MO'L has shown me I don;t need to bring half the wardrobe with me when I travel....did 2 weeks in the sun last year and a weeks skiing this year - both times with just the 10kg hand luggage.
What are the bets he'll drop the allowance down to 7kg soon as there were loads of people with the full allowance of hand luggae on the flight?