In front 5-3 at the interval the winners grabbed a crucial second try ninety seconds after the resumption by outhalf Tom Oâ00Neill whose conversion attempt came back off an upright.
Garry Bateman had put them in front after only three minutes and Crescent suffered a blow after 17 minutes when centre Michael Mulhihill was stretchered off and taken to hospital with a leg injury.
Diarmuid Dee made amends for an earlier miss when he landed a Crescent penalty after 23 minutes.
In the second half Crescent went all out for a try but Oâ00Neill made sure for St Munchinâ00s with a late penalty.
St Munchinâ00s had a player sin-binned ten seconds from the end but by that stage they had ensured a final meeting against PBC at Thomond Park next Wednesday.
Scorers: St Munchin's: Tom O'Neill try, pen, Gary Bateman try.
Crescent Comp:Diarmuid Dee pen
St Munchinâ00s College: D Morrissey, C Shanahan, A Hannigan, COâ00Donnell, G Bateman; T Oâ00Neill, B Nugent; E Kelly, G Sheehan, D Byrne; M McSweeney, (rep B Molloy, 46 mns). B Kilkenny; G Powell, S Airey, M Corbett.
Crescent Comp: G Barry, C Beatty, M Mulvihill (rep S Ryan, 17 mins), D Dee, E D Walsh; P Liston (M Ryan, 45 mins), R Doherty (G Lyons, 40 mins),; J Sheehan, (J Keyes, 44 mins), J Green, S Gleeson; B Mulcair, E Fitzgerald; K Hanley, D Culhane, J Dineen.
Referee: D Moloney (M.A.R.)