Richie Lane’s pinpoint place-kicking proved vital again for Clontarf as they came away from Dooradoyle with four valuable Division 1A points.
These sides' last Ulster Bank League meeting – a 15-14 win for Garryowen at Castle Avenue last year – was effectively decided by two first half tries.
But this time around, defences held sway and it was left to the respective place-kickers to fight it out for supremacy on the scoreboard.
Winger Richie Lane, who scored 26 points in his previous two outings, kicked Clontarf ahead with a tenth minute penalty goal.
He added another midway through the half and a third as half-time loomed, with Aaron Dundon, Ben Reilly and Barry O'Mahony impressing up front for 'Tarf.
Garryowen, who had Munster's Declan Cusack and Mike Sherry in their side, had to wait 36 minutes for their opening score from Cusack.
The out-half's penalty strike reduced the arrears to 9-3 at the break. Winger Alan Gaughan tagged on a second penalty for the Light Blues, but that was as close as they came in a game that failed to produce a try.
Clontarf, with New Zealander Andy Wood back at the helm, remain at the top of the table following their third straight win. Garryowen's second defeat has them sixth overall.
Referee: Paul Haycock (IRFU)