The Waterford man bagged 0-14, including ten frees, as the eight-time winners set up a last four date with University of Limerick or Limerick IT at Queen’s next week.
However DIT will look back with regret on two missed penalties in the opening 60 minutes
Sean McGrath’s first half effort was stopped by WIT full-back Pauric Gahan and goalkeeper Stephen O’Keeffe, excellent again between the sticks, kept out McGrath with just six minutes remaining.
O’Keeffe’s late error allowed his opposite number Darren Gleeson to net for Tipperary at Semple Stadium last Saturday night.
But Déise boss Derek McGrath backed O’Keeffe to bounce back and he did so with a couple of blinding saves to deny McGrath from play at Parnells GAA club.
At the other end, Mahony was the star turn but the game’s crucial score was a goal from Tipperary’s Liam McGrath just seconds into extra-time.
O’Keeffe was beaten once, by Diarmuid Horan in the first half, as DIT, inspired by captain Kieran Bergin, led 1-6 to 0-8 at half-time.
A tight second half was level five times and finished deadlocked (1-10 to 0-13) when Wexford’s Liam Óg McGovern clipped over a 56th minute equaliser for DIT.
McGrath netted early in extra-time for WIT and last year’s beaten semi-finalists kicked on from there to lead 1-17 to 1-12 at the turn. And with Mahony on song, WIT went on to record a seven-point win.
Coach Colm Bonnarreflected: “We knew DIT topped the group, they were there on merit. We came second in our group, had to travel away and we knew DIT were going to throw everything at us.
“We’re delighted to come away with a hard-fought win. In normal time, especially in the first half, they looked like they were going to creep away from us. But lucky enough we saved two penalties and came down the field and tacked on a few points.The goal we scored in extra-time was the difference.”
Scorers for DIT: S McGrath 0-5 (4f), K Bergin 0-4, D Horan 1-2 (1f), M Moloney (f), N McMorrow & L Óg McGovern 0-1 each.
Scorers for WIT: P Mahony 0-14 (10f), L McGrath 1-1, C Kenny 0-3, H Kehoe, J Dillon (sl) & G O’Brien 0-1 each.
DUBLIN IT: F McGarry (Dublin); W Devereux (Wexford), G McManus (Dublin), F Power (Dublin); C Robinson (Dublin), L Rushe (Dublin), M Moloney (Kilkenny); J Henderson (Wicklow), S Morris (Meath); T Connolly (Dublin), K Bergin (Tipperary), D Horan (Offaly); N McMorrow (Dublin), L Óg McGovern (Wexford), S McGrath (Dublin).
Subs: J Hobbs (Wexford) for McManus (33), O Boland (Dublin) for Morris (56), P McDonagh (Cork) for Connolly (70).
WATERFORD IT: S O’Keeffe (Waterford); G Teehan (Kilkenny), P Gahan (Kilkenny), J Maher (Waterford); T Hamill (Tipperary), M Power (Kilkenny), S Murphy (Wexford); S Roche (Waterford), C Kenny (Kilkenny); H Kehoe (Wexford), P Mahony (Waterford), E Murphy (Kilkenny); L McGrath (Tipperary), J Dillon (Waterford), G O’Brien (Waterford).
Subs: M Gaffney (Kilkenny) for Power (42), J Langton (Kilkenny) for Roche (49), A Kenny (Wexford) for Kehoe (59), L Hickey for Teehan (60), Kehoe for Gaffney (60), C Heffernan (Waterford) for McGrath (65), J Hayes (Kilkenny) for Kehoe (73).
Referee: B Kelly (Westmeath).