Replacements: Dave Rowley, Peter O'Shea, Darren Mulcair, James McInerney, Eamonn Connolly.
Replacements: Alan Trenier, Adam Howard, Killian O'Neill, Fergal Lawlor, Peter Robb.
The result, coupled with Clontarf's home win over Ballynahinch, meant that the north Dubliners finished just a point ahead of Old Belvedere at the end of a memorable 18-match campaign.
'Belvo's final round opponents Garryowen only had pride to play for after suffering relegation for the first time in the club's history.
But the Light Blues registered tries through outstanding winger Harry McNulty (2 minutes) and flanker Anthony Kavanagh (38) to give them a deserved 14-7 half-time advantage.
Old Belvedere fought on despite struggling to create openings and a drop goal and penalty from Garryowen out-half Andrew O'Byrne sandwiched a Josh Glynn penalty at the other end.
Paul Cunningham's charges did close the gap to 20-15 thanks to a Philip O'Dwyer try after Garryowen forwards Ed Rossiter and Aaron McCloskey both saw yellow.
Young winger O'Dwyer himself was sin-binned in the closing stages and O'Byrne coolly slotted the resulting penalty.
Glynn responded in the 78th minute but the Belvedere push for a late match-winning try was thwarted by Garryowen's resolute defence.
A nightmate scenario played out for the odds-on favourites who journeyed south to Limerick on the cusp of winning their second league crown in four seasons.
The Belvedere nerves were obvious early on as Leo Auva'a knocked on from the kick-off and Garryowen struck for a try off the resulting scrum.
There were just 86 seconds on the clock when O'Byrne's drifting pass sent big winger McNulty ghosting through a gap with the 'Belvo midfield ruthlessly exposed. O'Byrne followed up with a pinpoint conversion from the right.
The visitors' powerful scrum won them a penalty in their own half, but a sidestepping run from O'Byrne almost played in Alex Wootton for the second Garryowen try - Aaron Sheehan came to Belvedere's rescue with a well-timed tap tackle.
Although O'Byrne screwed a kickable penalty wide in the 12th minute, Tom Tierney's side continued to dominate both possession and territory as the first quarter came to a close.
Steve Crosbie, after a couple of miscued touchfinders, finally got 'Belvo into the 22 and they pressed from a loose Garryowen lineout with O'Dwyer tackled into touch near the left corner.
They exerted more pressure as Garryowen coughed up possession in their 22, and Auva'a tapped a penalty to release his back row colleague Kieran O'Gorman to score against his former club.
Glynn added a fine conversion from the left to bring the Dubliners level, with the table toppers pressing again soon after as Auva'a and prop Pat O'Regan got over the gain-line.
An incisive midfield run from Cian O'Shea saw Garryowen threaten at the other end and although he was hauled down, the Limerick club built once more to run in a second try just before the break.
The strong-running McNulty and influential number 8 Mark Rowley - the eventual man-of-the-match - featured in the build-up and numbers out wide on the left allowed JP Cooney pass back inside for Kavanagh to finish off.
O'Byrne managed to add the extras for a seven-point interval lead and he landed a drop goal six minutes into the second half to keep Garryowen on course.
Glynn was released on the left wing to set up a 52nd minute penalty which he converted himself with a typically accurate kick.
As the end-to-end nature of the game led to tired bodies and more openings, Garryowen moved 10 points clear approaching the hour mark.
A wonderful weaving run from back rower Rowley was rewarded by three more points from O'Byrne's reliable right boot.
Nonetheless, referee Gary Conway increased his influence on proceedings as a series of penalties led to Garryowen having two members of their pack binned.
Hooker Rossiter was the first to go in the 58th minute having been caught coming in at the side of a ruck. Second row McCloskey followed him six minutes later for another ruck offence.
Belvedere's second try was not long in coming after that as Aidan Wynne and O'Gorman stretched the home defence on the right before the ball was worked out to the opposite flank where O'Dwyer used his speed and strength to make the line.
Unfortunately for the vocal travelling support, Glynn pushed his conversion attempt to the right and wide.
Still, Garryowen's lead had been whittled down to just five points - 20-15 - as they scrapped furiously for only their fourth league win of the season.
In a key moment, their hard-working blindside Shane Buckley offloaded brilliantly for replacement Darren Mulcair to take play back into the 'Belvo 22 and hands in the ruck led to O'Dwyer being sin-binned.
That 72nd minute infringement made it 14 players each and O'Byrne claimed the three points on offer to widen the margin to eight.
Time was not on 'Belvo's side now, but Jonathan Slattery and his team-mates gave it their all over the remaining minutes and a Glynn penalty from the left brought them back to within a converted score.
They bossed possession against 14-man Garryowen, working it wide for the likes of the Leinster-capped Auva'a and former Connacht back Wynne.
The visitors did manage to get back into the Garryowen 22 with the posts drawing them ever closer, but the defiant Light Blues forced a final penalty at the breakdown and Old Belvedere were left crestfallen by their final day collapse.
Referee: Gary Conway (IRFU)