Rockwell got off to a brilliant start when their outstanding scrum-half Ben Riley raced in under the posts after nine minutes and Bill Johnston added the extras.
Amazingly, their only other score came with just a few minutes to go — a Johnston penalty.
In between, Ardscoil dominated and after 14 minutes flanker Jerry McCabe picked up possession and swept in for their first try.
Their second, after 28 minutes, was a superbly taken effort by the excellent Kelvin Brown following a break by Shane Fenton and Tom Walsh. David O’Mahony converted Brown’s try and also kicked a penalty in each half. They now advance to play PBC in the last four.
ARDSCOIL RÍS: L Brock; P De Brun, P McNamara, J O’Donnell, L White; D O’Mahony, H Bourke (D Scannell (57); P Lyons, K Bracken, T Walsh; D Ryan, K Brown; J McCabe, J Sexton (D McMahon (57), S Fenton.
ROCKWELL COLLEGE: C O’Shaughnessy (S Lowry ht); S Quirke, L Molloy (E O’Sullivan 58), W Coffey, E Stone (S Murphy 48); B Johnston, B Reilly; J Tierney (C Casey 39), J Bolger (T O’Keeffe 58), D O’Dwyer; M Flanagan (C Cashman 38), W Purcell; S O’Connor, D Fehilly (I O’Reilly inj 27), T Anglim.
Referee: P O’Connor, (MAR).