Replacements: Ed Rossiter, Peter O'Shea, Mark Melbourne, Andrew O'Byrne, Darren Mulcair.
Replacements: Scott Flanagan, Arthur Lyons, Mark Fallon, Philip Brophy, Robbie Glynn.
For their part St. Mary's, who had Mark Fallon, Darragh Frawley and Kevin Sheahan sin-binned at different stages, will be disappointed that they could not manage a fourth try.
Jamie Gavin had Garryowen in front with a penalty after 12 minutes but they fell behind when they bought a dummy which saw Leinster's Darren Hudson scoot through for a converted try which was set up by captain Darragh Fanning.
Garryowen's defence slipped up again a few minutes later as the dangerous Hudson completed his brace, with Fanning involved in the build-up once more.
Out-half Gavin Dunne added the extras and in between the tries, Gavin kicked two penalties for the Light Blues.
A few minutes into the second half Gavin landed his fourth penalty to leave only two points between them, but a penalty by Dunne was a timely boost for St. Mary's.
They wrapped up the points when Fanning - another of their Leinster-capped brigade - raced over after a great break by Dunne who kicked his third conversion.
Entering the final quarter, Gavin kicked his fifth successful penalty for Garryowen who face the prospect of losing their top flight status for the first time since the league began.
Giving his reaction to the Limerick club's ninth successive Division 1A loss, forwards coach Paul Neville admitted to the Irish Examiner: "We're in dire straits. We just need to stick together as a group. It's not easy.
"Today was the first time we nearly had a decent squad with guys coming back and I thought the effort of the pack was outstanding. We're finding it very difficult to score tries, we're working very hard to get into positions and then letting ourselves down.
"Is it mathematically possible for us to get ourselves into a play-off...I don't know but it's looking worse and worse. The way we lost the Munster Cup final recently (in extra-time to Cork Constitution) was crushing.
"We were devastated after it and it had a knock-on effect. When you're losing games and you need a rub of the green and something like that comes along on top of it, it was like kicking a fella when he's down."
Referee: Gary Conway (IRFU)