The Ballygunner star, who captured a Waterford SHC medal last weekend, made light of poor conditions to collect the massive haul at Cashel’s Leahy Park yesterday.
Mahony scored 1-4 in both halves to hand St Flannan’s their first defeat of the competition this year.
Niall Arthur shone for St Flannan’s in the first half with 1-7 but the Clare outfit managed just three further points.
Mahony, a former Harty Cup medallist, hit 2-2 from play while Dunhill corner forward Eamon Murphy had 1-1.
De La Salle were fortunate to get past county rivals Waterford Colleges in their first match and yesterday, they had to come from behind to collect the win.
Murphy fed Mahony for the first of his two goals after 20 minutes but five minutes later, Arthur responded in kind to hand wind-assisted St Flannan’s a 1-3 to 1-2 lead. Shane O’Connor shot the Ennis side two points clear before a brace of Mahony frees tied the game at 1-4 apiece at half-time.
Disaster struck for St Flannan’s three minutes after the restart when Mahony struck for his second goal, pouncing on an error by substitute Conor O’Loughlin, who had replaced captain Paul Flanagan at half-time.
Mahony’s goal handed De La Salle a lead they would hold all the way to finish and the game was decided as a contest in the 42nd minute when DLS scored a third goal. Murphy cracked home a rising drive past Niall Woods and when Mahony added a point from play, DLS led 3-7 to 1-5.
St Flannan’s twice cut the gap back to five points but DLS comfortably held out to assure themselves of a place in the knockout stages of the competition that they won in 2007 and 2008.
DLS will prove difficult to beat once again this season as in-form Mahony now turns his attentions to next Monday’s East Waterford U21 final against Passage East, with a county final place on offer for the winners.
Scorers for St Flannan’s: N Arthur 1-7 (0-5f, 0-2 65s), S O’Connor, D O’Halloran & M O’Neill 0-1 each.
Scorers for De La Salle: P Mahony 2-8 (0-6f), E Murphy 1-1, R Spencer 0-1.
ST FLANNAN’S: N Woods; P Flanagan, G O’Connell, C Doolan; C O’Gorman, B Hehir, S O’Halloran; J Guilfoyle, C Malone; E Malone, A Considine, T Kelly; S O’Connor, N Arthur, D O’Halloran.
Subs: C O’Loughlin for Flanagan (ht), M O’Neill for C Malone (41), J Colleran for D O’Halloran (60).
DE LA SALLE: P Cooke; S O’Keeffe, A Walsh, D Sutton; K Walsh, E Madigan, D Kearnes; J Hayes, J Dillon; J Byrne, P Mahony, H Barnes; E Murphy, R Spencer, B Fitzgerald.
Subs: C Morris for Hayes (ht), A O’Sullivan for Spencer (60).
Referee: John ‘Bob’ Ryan (Cashel).