Clontarf were happy campers as they exited Tom Clifford Park after a doughty Division 1A win that saw them come from behind early on.
The game was only two minutes old when Clontarf were penalised for not releasing and Young Munster's leading scorer this season, scrum half Brian Haugh, popped over the penalty.
And it got worse for the visitors three minutes later. They set up a promising three-quarters move deep in Young Munster territory but Cookies winger Darragh O'Neill made an interception and raced from his own 10-metre line to touch down in the corner.
8-0 up inside five minutes and things could not have been much better for Mike Prendergast's side. They made it 11-0 shortly afterwards when Haugh converted his second penalty, this time for offside.
But Clontarf got back into the game when Richie Lane struck a penalty into a strong wind after Young Munster did not release possession and only a point separated them after 30 minutes.
Following a scrum, the ball was knocked from the hands of Haugh as he was tackled and wily blindside Martin Garvey got in for a timely try. It was 11-10 at the break.
Two minutes into the second half, Clontarf grabbed their second try. Several players handled before full-back Timmy McCoy, taking the final pass from a fast-breaking Michael McGrath, got in at the corner to give the north Dubliners an unlikely 15-11 lead.
Lane closed the game out with another penalty ten minutes from the end, sealing Clontarf's second successive league win over Limerick opposition – they bagged a bonus point against UL Bohemians last week.
Munsters, who had a disappointing final hour in wet and windy conditions, will have to regroup quickly for next Friday's derby clash with UL Bohs, while Clontarf are back on home turf to host current table toppers Lansdowne.
Referee: Alain Rolland (IRFU)