Dundalk super sub Tiernan Mulvenna fired home a 90th-minute goal to give his side a 1-0 win over a shell shocked Derry City.
With league leaders St Pat's not in action, Derry had the chance to go level on points with them at the top but they were unable to find a way past a stubborn Dundalk defence.
Both sides looked like they were going to cancel each other out Mulvenna won the points for the visitors with a fine finish in injury time.
The Brandywell men made four changes to the side which defeated Bohemians in the league last week.
Leading scorer Rory Patterson along with Michael Duffy, Ryan McBride and Barry Molloy came into the starting line-up, with Mark Griffin, Paddy Kavanagh and Michael Rafter all dropping to the bench and Ruaidhri Higgins missed out through injury.
Dundalk started brightly with lone striker Patrick Hoban causing Ryan McBride a host of problems and on 12 minutes he got away from the City centre-back before picking out Kurtis Byrne, but he fired wide from acute angle.
Minutes later a fantastic low drive by Richie Towell from 25 yards saw City goalkeeper Gerard Doherty make a super out-standing save.
On 20 minutes the home side missed a glorious chance when Patterson charged down Peter Cherrie’s clearance, but the Dundalk net-minder got back on his line and blocked the striker’s close range drive.
City had to make two changes before the break with Patrick McEleney and Kevin Deery both going off injury and being replaced by Patrick Kavanagh and Michael Rafter.
The visitors themselves were forced into a substitute right on the stroke of half-time when Chris Shields hobbled off with a groin injury and was replaced by Stephen McDonnell.
Derry started the second half on the front foot and early in the half a clever flick on by Patterson found left winger Michael Duffy, who did well to get away from John Sullivan, but his left footed shot from just inside the box, rattled Cherrie’s crossbar.
Dundalk had a glorious chance to break the deadlock on 72 minutes when Richie Towell picked the ball up on the half-way line, before running at the City defence and cutting the ball back to Darren Meehan, but his effort from six yards was bravely blocked by Ryan McBride.
A fantastic through ball by Eoghan Osborne found substitute Mulvenna and with virtually his first tough he made no mistake firing low past an out-rushing Doherty.
Derry City: Doherty, Madden, Greacen, McBride, S McEleney; Deery (Rafter 39), Molloy, B McNamee; P McEleney (Kavanagh 30), Patterson, Duffy.
Dundalk: Cherrie, Boyle, Meehan (Mulvenna 89), Massey, Osborne; Dillon, Shields (McDonnell 46), Sullivan, Towell, Byrne, Hoban (Faherty 80).
Referee: Mr Rob Rogers (Dublin).