Ardscoil Ris made it a Limerick double over Cork schools in a stunning climax at the Mardyke.
Five minutes into injury-time, CBC led 19-12 and were looking good value for the win until Ardscoil replacement Nathan Randles crashed over.
Conor Fitzgerald converted to leave the sides locked at 19-19 and Ardscoil advancing as a result of scoring the first try.
Ardscoil full-back Darren Gavin went over in the third minute, but Christians responded with a try from prop Conor Leahy.
James Brennan put Ardscoil back in front, but Christians went ahead through Ben O'Callaghan. O'Callaghan's second try, into the second half, put Christians five ahead, but they were denied late on.
ARDSCOIL RÍS– D Gavin (N Randles 55); M Fitzgibbon, P Collins, J O'Donnell, J Brennan; C Fitzgerald, J Moore; P Lyons, T Chan, J O'Shea; C Cannon (T Richardson 50), R Coffey; O Bernard, J Sexton, J McCabe.
CBC – I Dorney (R Cassidy 23); B O'Callaghan, A McHenry, O Conway, A O'Brien; M O'Halloran, R McGuire; G Maloney, F Hennessy, C Leahy; D O'Donovan, C Penney (D Barry 37); E Feeney, E Cahill, D McGinn.
Ref – C McTaggart (MAR).