Dolphin led 17-7 at half time and looked to be cruising to victory when out-half John O'Mahony landed his second penalty just three minutes after the restart.
However, Crescent came racing back, replacement PJ McLoughlin's try was converted by Eoin Reddan (20-14) and when Matt T'Pau crossed again in the 23rd minute, Dolphin looked vulnerable. Reddan's conversion of T'Pau's try went narrowly wide, O'Mahony steadied Dolphin nerves with his third penalty and then tagged on two more to seal the outcome.
Dolphin. D Keeshan; J Purcell, B O'Neill, M Knowles, D Whelan; J O'Mahony, D O'Mahony; E Knowles, D O'Farrell, J O'Sullivan, D Pomeroy (capt), R McGrath, S Madigan, D Murphy, J Coughlan.
Old Crescent. J Cusack; A O'Loughlin, S Stephens, M Brodie, F Hogan; M Mullins, E Reddan; S Murphy, L Browne, J Cullinane, I Bloomer, P Ryan, D Murphy, M T'Pau, D McDonagh.