A first-half try James McKeogh and two second-half scores by Tony O'Sullivan and Jack Delaney were enough for Crescent to set up a final place against CBC or PBC.
Ronan Kiely responded with a consolation try for Castletroy five minutes from time but Crescent were well worth their victory.
Crescent got off to the perfect start to the game when Andrew Shanahan's clearing kick was blocked down by David Rickard and McKeogh was left with the simple task of scoring with Delaney adding the extras for a 7-0 lead which they held to half-time. Crescent used the elements well in the second half and were rewarded when O'Sullivan broke through for his try.
Crescent CC - C Quilligan; G Kelly, S Hurley, T O'Sullivan, B Cosgrove (B Davey, 53); J Delaney, J Connolly; E McNamara, J Fitzgerald, J Cross (B O'Sullivan, 58), D Doyle (A Herbert, 58), S Malone, B Scott, J McKeogh (O Evans, 51), D Rickard.
Castletroy College - R Murphy; D McDonagh (P Ross, 55), J Aylward, A Rowsome (S Hanrahan, 49), L Cunneen; A Shanahan, M O'Hanrahan; J McCormack (S O'Neill, 50), R Magill, S O'Shea, J Moloney (J McNamara, 54), O O'Shea (R Kiely, h-t), S Quirke, C White, D McMahon.
Ref - T O'Donoghue (MAR)