Shannon slipped to their first defeat in nine AIB League games as St Mary's College triumphed at a wet and windy Templeville Road.
Mary's clung on for only their second win of the league run thanks to some heroic defending.
Leinster's Jonathan Sexton kicked an early penalty before a series of scrums led to Hugh Hogan notching Mary's only try, 15 minutes in.
Shannon captain David Quinlan touched down soon after, but Andrew Thompson's 51st-minute penalty was the game's only other score.
Ballymena put in arguably the performance of the day as they whitewashed Lansdowne at a rain-lashed Eaton Park.
Jacques Benade's side managed to keep their opponents to nil for the third week running - UCD (19-0) and Greystones (27-0) were accounted for in recent weeks.
A 21st-minute try from winger Chris Browne, set up by scrum-half Craig Woods, got Ballymena up and running.
Ulster centre Seamus Mallon sliced through a gap to grab the clinching try, nine minutes from time.
Terenure College lost for the third week running in the AIB League as UCD took the spoils at Belfield today.
Winger Cian Aherne grabbed his second try in as many games and Leinster prop Ronan McCormack also touched down for the victorious UCD side.
They led 13-8 at the break with Terenure replying to Aherne's try with a touchdown from flanker Brian Moroney.
David McAllister converted a second 'Nure try from flanker Peter O'Malley but it was not enough to deny the hosts.
Connacht's Liam Bibo kicked 13 points as Galwegians condemned bottom side Greystones to their fifth loss of the league campaign.
The former Australian Under-21 international converted tries from lock Andrew Browne and winger John Cleary as 'Wegians ended a run of three defeats.
'Stones led 12-7 at half-time with centre Aaron Higgs touching down under the posts and back rower John O'Beirne also powering over.
But the visitors turned it around with Cleary's opportunistic score and rounded off the win with Bibo's third penalty.
Garryowen handed Dungannon their fifth successive defeat of the Division One campaign at Dooradoyle.
The defending champions leapt from seventh to sixth in the standings as tries from Keith Earls (2) and Ciaran O'Boyle propelled them past John Andrews' side.
Conan Doyle sent Earls over for his first try, eight minutes in. O'Boyle's late effort guided Garryowen to a 12-0 interval lead.
Earls combined with Federico Quaglia for his second try and Dungannon undoubtedly deserved more than just a 56th-minute score from David Parkhill.
Cork Constitution kept up their unbeaten start to the Division One campaign by powering past UL Bohemians at Temple Hill.
In a repeat of last season's semi-final, Con held all the aces and the win sees them reclaime top spot from Shannon.
Two Jeremy Manning penalties sandwiched an intercept try from centre Tom Gleeson as Con turned around with a 13-3 buffer.
Bohs' only points came from a Fergal Lawlor penalty, while full-back Denis Hurley finished off a smashing move for Con's second try.
Clontarf edged themselves back into the play-off picture by coming from behind to beat in-form Dolphin at Castle Avenue.
The north Dubliners trailed by 12-8 after four well-struck penalties from Dolphin out-half Barry Keeshan.
'Tarf notched a breakaway try through winger Philip Howard in the first half but Keeshan's place-kicking had the visitors 9-8 ahead at the break.
In a forwards-dominated game, 'Tarf turned the screw in the second half with tries from winger Max Rantz-McDonald and out-half Ian Keatley.
Blackrock College made use of a first half wind advantage to see off the challenge of Old Belvedere at Stradbrook this afternoon and get back to winning ways in the AIB League Division One.
'Rock moved up one place in the Division One table to ninth as they outfoxed their south Dublin rivals.
A 17th-minute try from centre Brian Canavan set 'Rock on their way, with lock Paul Huntley's late effort handing them a 17-3 half-time advantage.
Old Belvedere out-half Richie Murphy muscled over for a try and kicked a penalty but Blackrock were not to be denied.
Losers to Shannon last week, 'Rock wanted to make amends straight away and they were 10-0 to the good by the 31-minute mark. Backed by the elements, the Des Dillon-led hosts took control when Canavan made a midfield break and he had the strength and pace to make the whitewash.
Belvedere, who lost 44-19 on their most recent league visit to Stradbrook in 1998, managed to steady themselves with Murphy's boot coming into play and youngsters Felix Jones and Eoin O'Malley getting some decent possession.
Fionn Carr's 31st-minute penalty, which took his tally for the campaign to 61 points, was replied to by Murphy four minutes later. However, 'Rock had the final say of the half as a surge from their forwards led to Huntley crossing the line and full-back Carr converted for a 17-3 score-line.
Belvedere got back in touch when Murphy emerged from under a pile of bodies to grab the visitors' only try of the afternoon. Yet with 'Rock's defence relatively water-tight, Murphy and company were restricted to just a penalty in the closing half-hour.
Murphy indeed was central to two players receiving yellow cards, midway through the second half.
The former Leinster out-half was caught lying on the wrong side of a ruck, 'Rock scrum half David Moore took offence and lashed out at the veteran and Belvedere lock Richie Leyden sought retribution, leaving referee John Carvill with little option but to sin-bin both Moore and Leyden.