McNulty seals Belvo berth in last four
Tries from Cian O'Sullivan and Sean McNulty at Donnybrook earned Belvedere a semi-final spot.
It was a high-intensity encounter, with bone-crushing energy put into every carry and hit.
St Mary's applied most of the early pressure, coming up one legal pass short of a try in the ninth minute.
The breakthrough eventually came in the 28th minute, when the Belvo forwards went on the front foot and the ball was swung back into the hands of left-wing O'Sullivan, whose sharp straight line sliced Mary's open.
James McKeown converted for a 7-0 lead, and that's how it stayed until the break.
On the resumption, a thrilling dart down the left by scrum-half Hugh O'Sullivan put Belvo right in the danger zone, but St Mary's stayed disciplined to clear their lines, with captain Niall Curran calling the shots.
They threatened, with centre Thomas Tierney showing neat footwork to make ground before full-back Matthew Oliver took him down.
Then Belvo passed up a kick at goal in favour of a five-metre line-out. It worked.
They were smooth in the air and powerful in the drive, and flanker McNulty was the boy in possession at the bottom of the maul for a 12-0 advantage in the 51st minute.
McKeown missed the conversion, but Belvo held on with few alarms.
Belvedere – M Oliver; G Farrell, J McKeown, H Sexton (J Iredale 31), C O'Sullivan; C Jennings (capt), H O'Sullivan; C Wynne-Walsh, S Osborne, T de Jongh; H Fenlon, B Egan; D McCaffrey, S McNulty, J Kenny.
St Mary's – S Coen; N Lynch (E Duggan 42), A Schuster, T Tierney, C Buckley; D Boyle, D O'Connor; R Fanning, O Donnelly, M McCormack; E Potts (L Duffy 28), N Curran (capt); R Foley, A McDermott, S Donohoe.
Ref – D Blake (Leinster Branch).