A STUNNING goal from Robbie Smyth in the 53rd minute proved pivotal as Longford sent Dessie Farrell's Dublin packing at Parnell Park.
After their resounding success over Carlow in the previous round, the reigning All-Ireland champions failed to reach those heights against a Longford team buoyed with confidence having recently secured the Hastings Cup.
The visitors deservedly took the lead when the lively Smyth kicked a fine score on the turn in the fourth minute.
Some errant shooting ensured Longford were left to rue the concession of a Dublin goal in the 20th minute, with Ciaran Kilkenny providing an exceptionally composed finish.
Emmet O Conghaile added a point within five minutes and Paul Maguire could well have extended Dublin's lead, but his blazed effort in injury-time was shanked wide.
The tightness of the first half was mirrored upon the restart as Smyth and Shane Carthy exchanged points.
But the breakthrough that Longford desperately sought came as Smyth delivered an unerring finish beyond Lorcan Molloy.
Substitute Gavin Ivory offered Dublin hope with two late points, but despite the sending-off of captain Padraig McCormack in the 58th minute, Longford clung on for a well-deserved victory.
SCORERS – Longford: R Smyth 1-3 (0-1f), R O'Connor, M Hughes (f), D Cooney 0-1 each; Dublin: C Kilkenny 1-1 (0-1f), G Ivory 0-2, E O Conghaile, S Carthy ('45') 0-1 each.
LONGFORD – P Columb; R Keogh, B O'Farrell, D Bennett; D Farrelly, P McCormack, D Cooney; P Thompson, D Gallagher; R O'Connor, B McKeon, D Mimnagh; R Smyth, M Hughes, J McGivney. Subs: C Lynn for O'Connor (51), A Dalton for McGivney (59), D Quinn for Mimnagh (60).
DUBLIN – L Molloy; R McGowan, David Byrne, N McGovern; E Lowndes, C Mulally, J McCaffrey; S Cunningham, E O Conghaile; P Maguire, S Carthy, Danny Byrne; P Mannion, A Caffrey, C Kilkenny. Subs: N Scully for Danny Byrne (25), C McHugh for Caffrey (38), G Ivory for Maguire (41), P Ryan for Mannion (44).
REF – G Whelan (Longford)