Blackrock will look back on this Leinster Senior Cup semi-final replay with a sense of dejection and irony. The first half, where they dominated territory and created most of the offensive play, was the period St Mary's scored their match-winning try. Not often does the winner come in the opening eight minutes.

But so too will Blackrock have to acknowledge St Mary's imposing defence, which served particularly well in the first half. Then, as Blackrock ran the ball towards the Old Wesley end in waves and St Mary's were fanning out, at times thinly, the question was how long before a leak appeared. St Mary's courageously answered the question.

But for the phases where St Mary's came looking for scores, Blackrock also held up their end on the back foot.

In the closing minutes of the opening period St Mary's props Michael Houlihan and Brian McGovern surged for the line in the left corner before outhalf Jonathan Sexton found himself, on the fourth recycle, wriggling over, only to be held off the ground by the cover as referee Tony Redmond blew for half time.

By then St Mary's had gained their precious seven-point lead. For that score, a lineout deep in Blackrock territory was confidently collected by second row Conor McInerney, who alongside Brendan Smith did a terrific job in that department.

The ball was moved left to right infield, where the recycle brought it back left again. Outside centre Stephen Grissing made the initial advance before full back Philip Brophy charged in as the defence were caught shifting direction.

From there on Blackrock squeezed. But they were unable to suck in numbers to the rucks and mauls and spread St Mary's across the pitch, and the half ended with Blackrock in full spate but with a missed penalty to rue and St Mary's leading.

The second half opened in the same vein. Blackrock outhalf Andrew Geraghty was flattened yards short, while blindside Diarmuid Laffan broke off to set up centre Brian Canavan, but he fell just short with the ball wretchedly falling out of his outstretched hand on the line.

David Blain kicked Blackrock's penalty on 42 minutes from an acute angle, but thereafter St Mary's looked increasingly dangerous despite the blow of losing openside Alex Hutchinson.

Sensing they had weathered the storm, they then set about running the ball and on 57 minutes winger Gary Roche looked certain of touching down in the right corner but just fumbled on contact when driving for the line.

Sexton, who kept Blackrock turning constantly with his ranging kicks from midfield, hit the upright with an excellent drop goal effort as St Mary's went through their brightest patch.

Blackrock then showed their defensive mettle but still needed points on the board. Frustrated, they spent the closing minutes turning on Sexton's long kicks to touch and - forced to run the ball from deep - never built the platform to threaten.

Scoring sequence: 8 mins: P Brophy try, J Sexton con 7-0; 42: D Blain pen 7-3.

ST MARY'S COLLEGE: P Brophy; G Roche, S Grissing, M Finlay, E Lernihan; J Sexton, B McDermot; M Houlihan, R Morris, B McGovern, C McInerney, B Smith, P Nash (capt), A Hutchinson, N Brew.

Replacements: M Aboud for Hutchinson (57 mins).

BLACKROCK: D Nyhan; D Blain, M Kavanagh, B Canavan, B O'Farrell; A Geraghty, C Willis; K O'Neill, J Coady, S McClafferty, S O'Dwyer, D O'Reilly, D Laffan, K O'Toole (capt), N Corkery.

Replacements: R O'Hara for O'Reilly (53 mins), C O'Daly for Geraghty (59 mins), R Stokes for Nyhan (72 mins).

Referee: T Redmond