Stephen O'Brien was the Longford hero when his penalty save in sudden death of a penalty shoot-out kept Longford in the Premier Division.
It was cruel luck on Harps who won a pulsating game 3-2 on the night thanks to a blistering hat-trick from Kevin McHugh.
Longford played the majority of the game with 10 men after midfielder Alan Reynolds was sent off for a second bookable offence.
It looked like Longford were going to be relegated when McHugh put Harps 3-1 ahead on the night with a goal in the secondhalf of extra-time.
However, the Midlanders levelled almost immediately when Paul McNally fired home a penalty following a foul on substitute Shay Zellor. The match then went to spot-kicks, and O'Brien's save from Tom Mohan's sudden-death penalty proved crucial.
Longford had taken a two-goal aggregate lead when Eric Lavine scored three minutes into the secondhalf.
Following Reynolds's dismissal, Harps got back on level terms on the night when McHugh beat O'Brien from close range. McHugh then sent the game into extra time with his second goal 10 minutes from the finish.
And in a pulsating finish Harps striker Brendan Devenney struck a post in the last minute of extra time.
West Brom manager Gary Megson has dismissed reports suggesting he is ready to quit The Hawthorns. Megson has guided the Baggies back to the top flight after an absence of 16 years, but was quoted as saying he was being asked to work with one hand tied behind his back.
Megson said "I am dismayed at some to the comments attributed to me to the extent I realise I am going to have to be much more selective in what I say."
FINN HARPS: Connolly, R Kenny, Sweeney, Fitzhenry, Bradley, Mohan, J Kenny, McGrenaghan, Devenney, McHugh, Boyle. Sub: Speak for Fitzhenry (50 mins).
LONGFORD TOWN: O'Brien , Holt, W Byrne, Perth, Reynolds, McNally, Kirby, S Byrne, Lavine, Coleman, M Byrne. Subs: O'Connor for M Byrne (62 mins). Zellor for Lavine (65 mins), H Kenny for O'Connor (115 mins).
Referee: J Stacey (Athlone).