Harps missed a late penalty and Longford survived a number of goalmouth scramble but still bagged the points with Kevin O’Connor the goalscoring hero.
Tommy McMonagle squandered a glorious chance to fire Harps ahead from close range in the opening minute but was denied by Ryan Coulter.
But Longford snatched the lead with a long-range lob from O’Connor that dropped beyond Richard Brush’s reach on 17 minutes. Brush was then forced to save from Don Cowan and Peter McGlynn.
Harps had a real let-off on 43 minutes when Eddie Dsane capitalised on some poor defending by Packie Mailey but shot wide.
Dsane was inches away from grabbing a second with a shot across the face of the Harps goal on 73 minutes. Harps were awarded an 80th minute penalty as Dave Scully was taken down in the box but Ryan Coulter saved Ruairi Keating’s poorly taken kick.
Brush; J. Mailey, Cowan, P. Mailey (Scully 51), McMonagle, Coll; Boyle, Harkin, Hanlon (McHugh, 82); Houston, Curran (Keating 66).
Coulter; Mulhall, Flynn, Mvita, Powell; Cowan (Gorman, 90), Dillon, O’Connor, Gannon, McGlynn (O’Sullivan, 77); Dsane.
R Matthews (Dublin)