Shannon suffered their second defeat of the Division One campaign as Clontarf edged them out at Coonagh.
Team captain Darragh O'Shea landed a 76th-minute penalty, steering Clontarf to their sixth straight league win.
Shannon number 9 Fiach O'Loughlin was whistled up for not rolling away from a tackle close to his posts and O'Shea stepped up to convert the match-winning score.
The sides were locked at 0-0 at half-time before flanker Heinrich
Stride touched down for Clontarf and Tadhg Bennett kicked two
penalties for Shannon.
Shannon: Pens: Tadhg Bennett 2
Clontarf: Try: Heinrich Stride; Pen: Darragh O'Shea
