Belfast Harlequins squandered a last-minute try-scoring opportunity against Bruff when the ball was knocked on in the act of scoring.
Michael Heaney kicked two first half penalties for the visitors who led 6-5 at the interval with the Bruff try being scored by Paul O'Brien.
Bruff missed two easy penalty chances in the closing minutes of the half but they ensured a 10-6 victory thanks to a try by replacement scrum half Andrew Cashman.
It followed a break by man-of-the-match Paddy Clery who offloaded at the right moment to enable Cashman become the match winner.
It was the Limerick side's second win of the Division 1B campaign and gives them a much-needed fillip heading into the Christmas break.
They should really have finished Harlequins off well before Cashman's clinching score, given their level of dominance in the second period.
Peter Malone's men had a couple of tries disallowed, but it was the Belfast outfit who took an early lead thanks to a third minute penalty from scrum half Heaney.
The home pack did most of the damage, drawing in the 'Quins defence before winger O'Brien was unleash out wide for a try on the quarter hour mark.
Heaney replied with his second penalty success, ten minutes later, and Adam Larkin's side remained in front with Bruff's Brian Cahill missing a penalty attempt from in front of the posts.
But the hosts improved their execution in the second period and after 'Quins replacement Alistair Heatlie was sin-binned, Cashman made the breakthrough. Credit to Clery and Eoghan Maher for their roles in the build-up.
The losing bonus point was enough to keep 'Quins in fifth place in the table, while Bruff, who remain second-from-bottom, are within reach of Galwegians, UCD and UL Bohemians above them.
Referee: Kevin Beggs (IRFU)