Tries by Cian O'Shea and Conan Doyle and a conversion and two penalties by Jamie Galvin gave the Dooradoyle side an 18-8 lead at half time.
Shane Buckley scored the home side's third try and Gavin converted and also added another penalty to secure victory over last season's champions.
Scorers Garryowen: Cian O'Shea, Conan Doyle, Shane Buckley tries, Jamie Gavin 2 cons, 3 pens.
St. Mary's College: Ciaran Ruddock, Matthew D'Arcy, Kurt Haupt tries, Conor Hogan pen, con.
Garryowen: Ronan O'Mahoney; Lorcan Burke, Matt Jarvis, Conan Doyle, Cian O'Shea; Jamie Gavin, Neil Cronin; Rory Brosnan, James Rael, Peter O'Shea; Alan Kennedy, Aaron McCloskey; Shane Buckley, Anthony Kavanagh, David Sherry.
Replacements: Dave Rowley, Brendan Cullinane, Conor Hartigan, Eamon Connolly, Geoff Ryan.
St Mary's College: Conor Hogan, Marcus O'Driscoll, Andrew Bourke, Rob Hudson, Steve Toal-Lennon; Philip Brophy, Matthew Darcy; Colm McMahon, Richard Sweeney, Robert Sweeney; Ciaran Ruddock, Gerry Sexton; Kurt Haupt, Gareth Austin, Kevin Croke. Replacements: Arthur Lyons, Tom O'Reilly, Darragh Keller, Kevin Sheehan, Joe Burns.
Young Munster 16, Lansdowne 15.
Young Munster ended a recent poor run with a narrow victory over leaders Lansdowne at Tom Clifford Park.
Four penalties from Craig Ronaldson had the leaders 12-9 ahead at half time with Brian Haugh replying with two for the home side and Mike Prendergast adding a drop goal.
Dom Lespierre proved the home side's hero as he scored a magnificent try under the posts which Haugh converted for a four point lead.
Ronaldson reduced the deficit in the closing stages with his fifth penalty, but a determined Munster's side held on for victory.
Scorers Young Munster: Dom Lespierre try, Brian Haugh 2 pens, con. Mike Prendergast drop goal.
Lansdowne: Craig Ronaldson 5 pens.
Young Munster: Craig O'Hanlon; Dom Lespierre, Mark Doyle, Lukas Kuntz, Darragh O'Neill; Mike Prendergast, Brian Haugh; Ger Flaherty, Ger Slattery, Hugh McGrath; Sean Duggan, Darren Gallagher; Neville Melbourne, Luke Russell, Mark Rowley.
Replacements: Daniel Montgomery, Darragh Cantillon, Colin Liston, Johnny Moroney, Garreth Quinn-McDonogh
Lansdowne: Ross McCarron; Cian Aherne, Foster Horan, Tom Daly, Matt Healy; Craig Ronaldson, Paddy O'Driscoll; Jack O'Connell, Tom Sexton, Martin Moore; Willie Earls, Rob Hynes; Tadgh Beirne, Charlie Butterworth, Clive Ross.
Replacements: Tyrone Moran, Richard Liddington, Steve Collins, Marcus Walsh, Sean Carey.
Referee: Dudley Phillips (IRFU)
Cork Constitution 23, Shannon 15
Shannon went down to their seventh consecutive defeat of the campaign as Cork Constitution kept their challenge alive with this victory at Temple Hill.
Johnny Holland put the home side ahead with a penalty after ten minutes and then converted a try by Conor Desmond ten minutes later to extend their advantage.
A second Holland penalty left it 13-0 at the interval.
Shannon rallied in the opening stages of the second half with a try, conversion and penalty by scrum half Rob Guerin.
A penalty and a conversion of a Gerry Hurley try by Holland restored the home side's thirteen point advantage. .
Cormac O'Donnell kept Shannon in the game with a late try but Guerin failed to convert which would have at least given the visitors a losing bonus point.
Scorers Cork Constitution: Conor Desmond, Gerry Hurley tries, Johnny Holland 2 cons, 3 pens.
Shannon: Rob Guerin, try, con, pen, Cormac O'Donnell try.
Cork Constitution: Darragh Lyons: Cathal Quinn, Ivan Dineen, Scott Deasy, Robert Clune; Jonathan Holland, Gerry Hurley; John Ryan, Andrew Cudmore, Brian Cagney; Philip Donnellan, Brian Hayes; Cathal O'Flaherty, Brendan Cuttriss, James Ryan.
Replacements: Pat Galvin, Graham Lawlor, Brian O'Hara, Conor Desmond, Niall O'Driscoll.
Shannon: James Loxton; Cormac O'Donnell, Eric Moloney, Richard Mullane, Luke O'Dea; Tadgh Bennett, Rob Guerin; Marcus Horan, Duncan Casey, Gary McNamara; Fergal Walsh, Sean McCarthy; Mike Cullane, James Murphy, Ken Hanley.
Replacements: Jason Cronin, Kevin Griffin, Kieran Power, Willie O'Connor, Mike Darcy.
Referee: Nigel Correll (IRFU)
U.L. Bohemian 9, Dolphin 19.
Another defeat for U.L. Bohemian at Annacotty sees the Limerick side slip into the relegation area after suffering their fifth defeat of the campaign.
A penalty from JJ Hanrahan to one by Barry Keeshan had the sides level 3-3 at the break.
Two further penalties and a drop goal by Keeshan had Dolphin 12-9 ahead with the game in injury time. Ben Martin added two penalties for the visitors.
An intercept try in injury time by Harry Fleming which Keeshan converted denied the home side a losing bonus point.
Scorers U.L. Bohemian: Ben Martin 2 pens, JJ Hanrahan pen.
Dolphin: Harry Fleming try, Barry Keeshan 3 pens, drp gl, con.
U.L. Bohemian: Ben Martin; Darren Moroney, Mossie Lawlor, Niall Kenneally, Rory White; JJ Hanrahan, Cathal Sheridan; Tom Burke, Eddie Rossiter, Gary Walsh; Ed Kelly, Darragh Frawley; Conor Kindregan, Ian Condell, Jack O'Donoghue.
Replacements: Sean Henry, Alex Simpson, Keith McManus, Tim Rafferty, Rory Cunningham.
Dolphin: Denis Hurley; James Kiernan, Harry Fleming, Danny Barnes, John Gleeson; Barry Keeshan, Brian Slater; Christy Condon, Niall Scannell, James Cronin; Chris Rowe, Dave O'Mahony; Miah Cronin, Ryan Murphy, Brian O Callaghan.
Replacements: John Cleary, John Leahy, Rob O'Herlihy, Chris Bannon, Rory Scannell.
Ballynahinch 55, Bruff 3
Leaders Ballynahinch cruised to a seven try victory over Bruff at Ballymacarn Park.
Brian Carroll kicked a first half penalty for the visitors who trailed 21-3 at half time.
Scorers Ballynahinch: James Thompson, Chris Quinn, Aaron Cairns, Roger Mc Burney, Conor Carey, Stuart Morrow, Neil Faloon tries, Chris Quinn 2 pens, 4 cons, Blane Mc Ilroy 3 cons.
Bruff: Brian Carroll pen.
Ballynahinch: Adam Craig: Chris Quinn, Stuart Morrow, Robin Harte, Aaron Ferris; James Thompson, Blane McIlroy; Kyle McCall, Gavin Armstrong, Conor Carey; James Simpson, Chris Napier; Robert Greer, Paul Pritchard, Neil Faloon,
Replacements: Aaron Cairns, Roger McBurney, Johnny Madden, Toby Manus, Chris Stevenson.
Bruff: Brendan Deady; Robert Finn, Mark Cosgrove, Gary Leonard, Andrew Breen; Brian Carroll, Eoghan Maher; Ger Collins, John Hogan, Colm O'Sullivan; Maurice O'Connell, Mike McCarthy; Dumani Mtya, Danny Keogh, Paddy Cleary.
Replacements: Cathal O'Regan, Michael Lynch, Mike Carroll, Richard McAuliffe, Noel Forde.
U.C.C. 24, Queens University 26.
A dramatic finish at the Mardyke as a tenth minute injury time try by Queens winger Damon Hall earned the Belfast side their first win of the campaign with Ritchie McMaster converting from the touch line in this Division 2A clash.
College led 5-0 at half time with a try from Sylvian Raimondi and the home side were 17-0 ahead eight minutes into the second half with tries from Luke Duffy and Peter McCabe and a conversion from Sean Og Murphy.
The visitors rallied and two tries in a seven minute spell had Queens 19-17 ahead with the game entering injury time. Brian Quill put the Cork side back in front with a try which Murphy converted on eighty four minutes but then came Queen's final rally to take the spoils.
Scorers U.C.C.: S Raimondi, L Duffy, P McCabe, B Quill tries, S Og Murphy 2 cons.
Queens University: D Hall 2 tries, C Spence, B Alexander tries, R McMaster 3 cons
U.C.C.: J Holland; S Raimondi, L Duffy, D Horgan, C Barry; S Og Murphy, D Foley; P McCabe, K Stokes, B Scott; D O'Connell, B Quill; C Gallagher, J MacCarthy, W Ryan.
Replacements: M O'Mahony, C Slowey, D McSweeney, O Dennehy, R Barry.
Queens University: C Colvin; D Hall, R Stewart, S McIllwaine, R Adair; R McMaster, P Farrell; E Bowden, N Sinclair, L Bennett; J Heron, R McCaughey; R Clarke, R Whitten, B Alexander.
Replacements: R Winters, J Craig, T Johnston, H Harbison, C Spence.
Referee: D Connolly (IRFU).
Old Crescent caused a big shock with a 30-23 win over unbeaten leaders Banbridge at Rosbrien. Simon Malone, Larry Hanly and James Nolan were try scorers for Crescent who were also awarded a penalty try. Aidan McNulty kicked two penalties and two conversions.
A penalty try proved decisive in the Cork derby as Highfield defeated Midleton 10-6 at the Towns Park. Paddy O'Toole converted and also added a penalty. Paul Daly replied with two penalties for the home side.
Cashel went down to another defeat losing 37-14 to Cashel at Donnybrook. Brendan O'Connor and Giles Boland scored tries for the Tipperary side and Conor Quinlan converted both.
In Division 2B Nenagh Ormond kept their promotion hopes alive with a 16-12 home win over Skerries.
Stephen Carey and Karl Mackey scored tries for Nenagh and Donal Fogarty added two penalties.
Thomond are still searching for their first win after losing at home 24-222 to Boyne. Mark Ryan scored a try for Thomond and John Lillis added five penalties.
Sunday's Well picked up a losing bonus point when going down 23-17 at home to NUIM Barnhall at Musgrave Park. Shane O'Sullivan scored a try for the 'Well and Shane O'Riordan kicked three penalties and a drop goal.
Ulster Bank League Division 1A: Clontarf 14, Old Belvedere 13;
Cork Constitution 23, Shannon 15;
Garryowen 28, St. Mary's College 23;
U.L. Bohemian 9, Dolphin 19;
Young Munster 16, Lansdowne 15;
Division 1B: Ballynahinch 55, Bruff 3;
Belfast Harlequins 36, Malone 3;
Blackrock College 18, Ballymena 23;
Buccaneers 25, Dublin University 23;
U.C.D. 36, Dungannon 18;
Division 2A: City of Derry 13, Galwegians 21;
Corinthians 30, Greystones 8;
Midleton 6, Highfield 10;
Old Crescent 30, Banbridge 23;
Old Wesley 37, Cashel 14;
Seapoint 16, D.L.S.P. 16;
Terenure College 25, Bective Rangers 15;
U.C.C. 24, Queens University 26;
Division 2B: Ards 13, Instonians 14;
City of Armagh 17, Navan 10;
Clonakilty v Connemara Postponed;
Naas 35, Suttonians 19;
Nenagh Ormond 16, Skerries 12;
Rainey Old Boys 31, Sligo 0;
Sunday's Well 17, NUIM Barnhall 23;
Thomond 22, Boyne 24;