Second half tries from Robbie Hudson and Gareth Austen steered St. Mary’s College to their fourth successive victory in Ulster Bank League Division 1A.
Garryowen, who have been one of the main challengers to table-topping Lansdowne so far this season, enjoyed a fine start to the contest as Connacht's Matt Jarvis split the posts with a 35-metre penalty after just two minutes.
This was not the start that St. Mary's would have been hoping for as they aimed to continue their excellent run of form, but their response to falling behind was immediate.
Their full-back Gavin Dunne stepped forward to convert a similar penalty effort a couple of minutes later. In a lively opening, it was becoming clear that the accuracy of the respective place-kickers was going to be of vital importance to the outcome of this tie.
Jarvis was certainly showing his worth to the Limerick men, as he once again edged the visitors into the lead with a fine kick from distance ten minutes in.
Indeed, the dynamic number 10 had the opportunity to move Garryowen further ahead with a third kick at goal shortly afterwards, although he surprisingly came up short on this occasion.
This was a minor let-off for St. Mary's, but they used it to their advantage by forcing Garryowen onto the back foot.
The Dubliners enjoyed a period of intense pressure near the Garryowen whitewash, and an excellent break by centre Robbie Hudson just shy of the 20-minute mark paved the way for winger Steve Toal-Lennon to score the opening try of the afternoon.
Dunne easily converted, but even though the Templeogue outfit continued to develop some good passages of play for the remainder of the opening period, a third successful penalty from Jarvis on 33 minutes meant that their deficit has been reduced to the bare minimum – 10-9 – in time for the interval.
Dunne was presented with the chance to restore his side's four-point cushion just before the break, but he was off target with a difficult kick.
This made it possible for Jarvis to give Garryowen the lead for the third time with a routine penalty 12 minutes after the restart.
With try-scoring opportunities at a premium in the third quarter, this was a major boost to Garryowen's chances of securing a much-needed victory against last year's champions.
Dunne was proving to be just as reliable from the kicking tee for St. Mary's though, and his effort from a tight angle just past the hour cancelled out Jarvis' latest kick.
Into the closing stages. St. Mary's looked like they were going to be happier of the two teams come the final whistle when Hudson touched down to the left of the posts after 70 minutes.
Hugh Hogan's men were now eight points to the good, but a fifth Jarvis penalty offered Garryowen a lifeline with a couple of minutes remaining.
However, it was St. Mary's who had the momentum on their side in the dying moments, and a third try from flanker Gareth Austen in the 79th minute ensured that the hosts won out and lifted the Shay Deering memorial trophy.
Referee: Nigel Correll (IRFU)