Mark Sexton came off the bench to score two tries and see St Mary's crowned Ulster Bank League champions at Templeville Road.
Mary's went into the game knowing a better result than Clontarf would be enough as they went in search of their first title since 2000.
Young Munster were down to 14 men after only six minutes as Darragh O'Neill was sent off for a spear-tackle on Darren Hudson -- yet Willie Staunton gave the visitors the lead after 10 minutes from a penalty, and he added a second four minutes later. Ger Slattery then crossed the line after a good lineout position.
Mary's first points came just after the half hour from Gavin Dunne and Sexton's introduction after 50 minutes really turned the tide for them -- he crossed the line two minutes later, Dunne converting.
Staunton added another Munster penalty on the hour mark, but within 10 minutes Dunne had kicked two penalties to bring his team within three points -- and that man Sexton stepped up with the decisive score.
Scorers -- St Mary's: M Sexton 2 tries, G Dunne 2 cons, G Dunne 3 pens. Young Munster: G Slattery try, W Staunton con, W Staunton 4 pens.
A 70th-minute penalty from Noel Reid gave Clontarf a narrow victory at the Aviva Stadium -- but St Mary's win meant that the Castle Avenue outfit came up just short.
In a pulsating 80 minutes, Lansdowne raced into a 20-10 half-time lead thanks to tries from Matt Healy, Sean Carey and Craig Ronaldson. A Tim McCoy five-pointer kept Clontarf in touch.
Clontarf fought back, cutting the Lansdowne lead down to two with a Lane try before Lansdowne pulled back ahead with another Ronaldson try.
But Clontarf refused to let their heads drop and a 58th- minute converted try from Aaron Dundon put the tie firmly back in the melting pot and a Noel Reid penalty on 70 minutes was enough to give Clontarf victory.
Scorers -- Lansdowne: C Ronaldson 2 tries, 2 cons, 1 d-g, M Healy, S Carey try each. Clontarf: R Lane 1 try, 2 pens, 2 cons, T McCoy, A Dundon try each, N Reid pen.
Blackrock suffered defeat at home to Garryowen in this absorbing encounter, a result which relegate 'Rock from Division 1A.
Blackrock opened the scoring on two minutes as scrumhalf Marcus Walsh showed fancy footwork to give his side an early lead.
Three penalties from Fergal Lawler and a try from Adam O'Loughlin gave the visitors a 14-5 advantage as half-time approached, but 'Rock replied with another try, this time from Barry Kelly to leave Garryowen ahead 14-10 at the break.
A try by Richard Liddy just after the restart gave Blackrock a 15-14 lead. Garryowen regained the lead after two converted tries from Fergal Lawlor and Kieran O'Gorman midway through the half.
However, Blackrock hit back with a try from wing Mark Scott Lennon but their hopes were dashed when Alan Vaughan went over for another Garryowen try.
Scorers -- Blackrock: A Letham, M Scott-Lennon, M Walsh, B Kelly, R Liddy Try each, D Fitzpatrick con. Garryowen: A Gaughan, K O'Gorman, J Gavin try each, F Lawlor 3 pens, 2 con, try.
Cork Constitution 22
Old Belvedere 25
A fighting performance from Old Belvedere saw them keep alive their survival hopes with a deserved victory at Temple Hill. They now face Belfast Harlequins in a play-off to retain their top-flight status.
Playing with the advantage of a strong wind in the opening half, Belvedere found themselves behind after eight minutes when Con' winger Cathal Quinn got in for a try in the corner.
Gearoid McDonald hit back immediately with a penalty for the visitors, but two unconverted tries from Danny Riordan and David Mongan left it 15-5 at half-time in favour of the visitors.
Gerry Hurley reduced Con's deficit with a penalty two minutes into the second half and then came a three-try blitz in six minutes.
Leo Auva'a scored a try for Belvo' to extend their lead and McDonald converted. Tries by Brian O'Hara and Jonathan Holland both converted by Hurley levelled the scores.
Shannon secured their Division 1A status with this victory at Musgrave Park. Needing to win by at least the same as Old Belvedere, Shannon knew their status would be safe and had this game wrapped up by half-time as they led 16-0.
Barry Keeshan kicked a penalty early in the second half, but tries by John Colgan and Alan Bennie and a conversion from Bennett secured the points for the Limerick side.
UL Bohemian 34
Belfast Harlequins 14
UL BohS were crowned champions after this victory over table-toppers and long-time leaders Belfast Harlequins at Annacotty and the defeat suffered by Dungannon.
The home side flew into a 14-0 lead after only six minutes with tries by Rory White and Darren Moroney, both of which Ben Martin converted.
A try by Michael Allen and a penalty try both converted by Stuart Olding made it 14-14 before Martin had a try which he also converted. In the second half, Martin kicked two penalties and converted a very late and crucial bonus-point try scored by Sean Rennisson.
Scorers -- UL Bohemian: B Martin try, 2 pens 4 cons, R White, D Moroney, S Rennisson try each. Belfast Harlequins: M Allen try, pen try, S Olding 2 cons.
Bruff guaranteed their place in Division 1B with this thrilling victory over Buccaneers at Kilballyowen.
Five minutes from the end the visitors led 18-15, but a try from Mike Russell with two left edged it for Bruff, who also had tries from Brian Cahill and John Shine with Cahill converting one of the tries and kicking a penalty.
Two tries from Sam Coughlan-Murray proved crucial in Belfield yesterday as UCD maintained their Division 1B status, while Dungannon missed out on promotion.
Coughlan-Murray crossed over seven minutes in, but the Tyrone side responded well and two penalties from Michael Lawton edged them into the lead after 25 minutes.
A penalty from James Thornton, along with converted tries from Coughlan-Murray and Alan O'Connor had UCD 22-6 up at the break.
Tries from Ali Birch and Conor Gaston (both converted from Peter Nelson) got Dungannon back in the game, but tries from Richie Bent and Tom Fletcher closed the victory out for UCD.
Scorers -- UCD: J Thornton 4 cons, pen, S Coughlan Murray 2 tries, A O'Connor, R Bent, T Fletcher try each. Dungannon: P Nelson 2 cons, pen, M Lawton 2 pens, A Birch, C Gaston.
RELEGATED UCC took an early lead with a Luke Duffy penalty, but the home side struck back to take a 17-6 half-time advantage and then dominated the second period completely.
Ballymena ran in four more tries for a bonus point and with so much possession, the score could have probably been even more convincing.
Luke Marshall converted another two and UCC finished the season firmly entrenched at the bottom of the table with only two wins from 18 games.
Scorers -- Ballymena: S Mulholland pen, try, A Fraser, R Hackney, I Vance, M Irwin, C Fitzpatrick try each, L Marshall, 3 cons. UCC: L Duffy 2 pens.
THERE was nothing at stake in this final AIL game, but Ballynahinch took a 21-10 half-time advantage and appeared to ease up towards the close against Galwegians at Ballymacarn Park.
Harry McAleese was in fine kicking form as he converted a James Simpson try.
The Connacht visitors were slow to start, but Tadgh Leader contributed half of their points with two penalties and the conversions to both of their tries.
Scorers -- Ballynahinch: A Ferris, J Simpson, F Lynch, I Whitten try each, H McAleese 3 cons, pen. Galwegians: I Muldoon, B Murphy try each, T Leader 2 pens, 2 cons.