Christian Brothers Cork booked a quarter-final with Glenstal Abbey, after overcoming Castletroy in an ill-tempered match at Annacotty.
Referee Tony O'Donoghue was busy throughout, issuing four cards, including two red, following a 30-man brawl in the opening half.
After a wet and windy start, CBC were reduced to 14 men when Darragh Moloney saw yellow for a high tackle.
Despite this, they then took the lead, when full-back Chris McAuliffe rounded off an impressive move to score a try.
Minutes later both sides received red cards, when Rory Burke of CBC and Patrick Bermingham of Castletroy were both singled out following a 30-man melee.
The home side then scored on their first meaningful attack, winger Rhys Joyce latching on to a neat grubber kick to score.
Out-half Conor Linehan missed the conversion, but nudged his side in front with a spectacular kick into the wind before the break to leave his side 8-7 ahead at the break.
The second half started as the first had finished and CBC received their third card of the match, when substitute hooker Vincent O'Brien was sin-binned for deliberately stopping a quick penalty.
Out-halves Linehan and Tomas Quinlan swapped penalties soon after, giving Castletroy a slight one-point lead with just over a quarter of an hour to play.
However, with O'Brien out of the sin-bin, CBC began to dominate, and it was Tomas Quinlan's penalty with 12 minutes to go which deservedly won the game for Christians.
Castletroy College – P Butler (J Murphy 20), C Malone, M McCarthy, K Carroll, R Joyce, C Linehan, R Cunningham; C Bartley, L Cronin, A Ivan-Clancy (E Fitzgerald 65), P Bermingham, J O'Neill (R Larby), B Burns, D Kennedy (capt) (N Clohessy 70), R Murphy. Subs: E Fitzgerald, J Doyle, D Coffey, N Clohessy, R Larby, S Carey, S Hoare, J Murphy.
CBC – C McAuliffe (O Hanly 66), R Slater, M Clune, J Dowling, G Ryan, T Quinlan (C Monohan 60), R Foley; L O'Connor, D Fitzgerald, R Burke, S Gill, D Moloney, A Penney (V O'Brien 31), T Arrigan, A McCarthy (S O'Hanlon 60). Subs: V O'Brien, S Galvin, R Quirk, C Barry, J Murphy, S O'Hanlon, C Monohan, O Hanly.
REF – T O'Donoghue.