Yes I've done this, but haven't done so in a while, just read the rules and after you've done the first leg, log on and pay for the bag (be aware that from 1st Dec, Ryanair are going to smother us with kindness and bag charges may be decreased)
I had a friend who used to do this.... she would take a foldable bag out with her inside one suitcase and book both for return trip. But I thought this had not been allowed recently?
AFAIK Ryanair claim to be a "point-to-point" airline and don't do return flights so I don't see how they could have a problem with having different luggage requirements for different flights.
Thanks all. I'll still do my checkin to be sure it works before I do too much shopping. Bargains are not bargains if I have to pay a fortune to check in my bag!