Finn Harps claimed bragging rights in the north west as Ciaran O'Connor's 94th-minute winner saw the Donegal men beat Sligo Rovers at Finn Park.
Raffaele Cretaro had given the Bit O'Red the lead on 59 minutes before Kilian Cantwell struck for his first goal for Harps nine minutes later.
Referee Arnold Hunter was at the centre of the late controversy as he adjudged Tobi Adebayo-Rowling to have fouled O'Connor inside the area.
O'Connor saw his penalty pushed away by Rovers goalkeeper Micheál Schlingermann but the Dundalk loanee was alert to pounce on the rebound to record a big win for Harps, who now move four points ahead of their neighbours.
Sligo were first to threaten during a competitive but scrappy opening period. Jonah Ayunga capitalised on Cantwell's wayward cross-field ball but was foiled by a strong challenge from Harps skipper Ciaran Coll who did well to get across and divert Ayunga's close range drive at goal just past the quarter hour.
Rovers have profited in recent weeks from the outstanding performances of winger Kieran Sadlier and the former Peterborough man was close to gaining an assist from a throw-in as he found Chris Kenny close to the dead-ball line. Harps goalkeeper Ciaran Gallagher was sharp to fall at Kenny's feet to halt the Wexford man's charge.
The Finn Park outfit were almost the masters of their own downfall again on 24 minutes. Coll's lapse in concentration saw him concede possession to the bustling Ayunga who in turn supplied Cretaro down the Rovers right. Cretaro's quick turn inside saw him cut past his marker before teeing up Sadlier who mis-queued from a profitable position.
The Donegal side had to wait until the half hour before their first real opportunity arrived. Rovers were caught at sixes and sevens at the back when Coll's shot was blocked. The rebound then landed to Cantwell but again Harps were thwarted by an abundance of bodies in front of Schlingermann's goal.
The home side should have taken the lead early in the second half. O'Connor neatly slipped past the challenge of Mick Leahy before forcing Schlingermann into a strong save. Sean Houston pounced on the rebound but his weak attempt was easily gathered by the Rovers net minder.
Schlingermann was called upon again and the Mayo man tipped Jonny Bonner's curling 20-yarder over his crossbar.
Dave Robertson's charges hit the lead just before the hour mark, Cretaro seizing on a John Russell ball and finishing with aplomb for his first goal of the season.
Sligo have been found wanting at the back this year, having conceded 14 goals in their opening five games, and their defensive frailties were highlighted once again as their lead lasted just the nine minutes.
A Harps free-kick resulted in a scramble in the Sligo area, but when the ball eventually fell to Cantwell he made no mistake with his maiden strike for the Ballybofey men since his move from Galway in the winter.
Bonner fired across goal eight minutes from time as the game hung in the balance and the unmarked O'Connor missed the target from a tight angle moments later.
The game was decided four minutes into injury time, though. Schlingermann was equal to O'Connor's penalty but the latter was quickest to react to fire home for the winner.
Drama at the end of @FinnHarpsFC @sligorovers ! #loi #GreatestLeagueInTheWorld pic.twitter.com/HRZ48RPiKD
— Daithí (@Daithionaroll) March 25, 2017
Finn Harps: Ciaran Gallagher; Damian McNulty (Eddie Dsane 66), Ciaran Coll, Kilian Cantwell, Gareth Harkin; Caolan McAleer, Ethan Boyle, Jonny Bonner, Sean Houston, Danny Morrisey (Michael Funston 90+5); Ciaran O'Connor.
Sligo Rovers: Micheál Schlingermann; Tobi Adebayo-Rowling, Mick Leahy, Kyle Callan-McFadden, Liam Martin; John Russell, Chris Kenny, Craig Roddan, Kieran Sadlier; Jonah Ayunga (Mathew Stevens 71), Raffaele Cretaro (Mikey Place 86).
Referee: Arnold Hunter.
