Dolphin opened the scoring with a try from prop Eddie Knowles after James Coughlan had opened up the Crescent defence and captain Dave Pomeroy added a second with O'Mahoney converting both and adding a penalty.
But they were always going to give it a go and for the opening 20 minutes of the second half they hammered the Dolphin defence and scored two tries, the first from replacement PJ McLoughlin in the 54th minute. Then, with fifteen minutes to go, Shane Stephens scored their third try to leave just one point (20-19) in it and it was very much a case of game-on.
However, Dolphin rose to the challenge superbly and with Coughlan, Sean Madigan and Donncha Murphy providing the inspiration, they gave O'Mahony the field position to strike the crucial penalties.
Dolphin. D Keeshan; M Knowles, B O'Neill, M Manning (R Kelleher 48 min)), D Whelan (J Purcell 74 min); J O'Mahony, D O'Mahony (K Murphy 50 min); E Knowles, D O'Farrell, J O'Sullivan; D Pomeroy, R McGrath; S Madigan, D Murphy, J Coughlan.
Old Crescent. J Cusack; A O'Loughlin, S Stephens, M Brodie, F Hogan (D Fitzgerald 49 min); M Mullins, E Reddan; S Murphy (PJ McLoughlin 40 min), L Browne (A McMahon 75 min), J Cullinane, I Bloomer (D Bowles 44 min), P Ryan, D O'Malley, D McDonagh, M T'Pau. .
Referee: D Courtney (Leinster Branch).