Young Munster stumbled to their second consecutive defeat in the AIL league after going 12 points down to rivals Garryowen.
Garryowen laid the foundations for the victory in the forwards who were superb.
The visitors led by 10-08 at half-time thanks to a try from O'Sullivan, supported by the entire Garryowen pack.
Garryowen scored two more tries in the second half to seal the win.