Did you sign the Vehicle Licensing Certificate? When transferring privately, you sign this bit, the vendor signs his bit, and is meant to send it on its way. If you've signed it and he's not bothered sending it, he can't do much with it (as anyone else will see your signature there, if he tries to sell it twice).
Either way, I think you have a trip to the Gardai as your next step: either to declare the VLC lost (if he has sent it) or fraud (if he hasn't, but took your money). Hopefully the Gardai can give some idea of how best to proceed.