Ardscoil Ris 17, Glenstal Abbey 13.
Fly half Jack Stafford kicked two early penalties for Glenstal but a try by Kelvin Brown, converted by David Mahony put Ardscoil in front after 28 minutes and they never looked back.
A penalty by O'Mahony made it 10-6 at the interval.
A minute into the second half skipper and fullback, Stephen Fitzgerald, had another try for Ardscoil which O'Mahony again converted.
Max Ryan had a Glenstal try after 48 minutes, converted by Stafford, and even when Paul McNamara was sin-binned for the winners Glenstal could not capitalise.
Ardscoil Ris: S Fitzgerald; B Moriarty, P McNamara, J O'Donnell, l Brock; D O'Mahony, D Scannell; K Bracken, S Fenton, T Walsh; D Ryan, P Staff; J Cannon, J McCabe, K Brown.
Replacements: H Bourke for Scannell (58), T Chan for Walsh (68).
Glenstal Abbey: C Hogan; M Boland, C Og Healy, J McAleese, D Kenny; J Stafford, R Johnston; J Healy, M Ryan, G Fitzpatrick; A Donohue, B Clancy; G O'Hara, J Murphy, N Mulvihill.
Replacements: E Ryan for Fitzpatrick (50), J Hartnett for J Healy (56), B Cosgrove for Murphy (62), S O'Sullivan for Kenny (64), R Hogan for Johnston (67).
Referee; G Finn (MAR).
The quarter-final draw took place after the game and the fixtures are listed below.
Seat Munster Schools Senior Cup Quarter-Final draw
11th February - Castletroy College v Glenstal Abbey
12th February - Ardscoil Rís v CBC
12th February - Crescent College v PBC
13th February - St. Munchin's v Rockwell