And it was former Blues player Michael Rafter who came back to haunt the home club as his brace gave Darius Kierans’ side a deserved victory.
After the sides finished level at Oriel Park on Tuesday, much was expected of the First Division side. But they failed to turn up, especially in the second-half, as Paul O’Brien’s outfit put in a tame effort and could have no complaints.
As time ticked down at the end of the first-half, Dundalk struck for the lead goal when Chris Shields hooked the ball in from the right and Michael Rafter, the former Blues player, had all the time in the world to find the net.
Referee Padraig Sutton was the centre of attention three minutes into the second-half when Seani Maguire went down in the area under a challenge from Liam Burns but the official booked the attacker for diving before Peter Cherrie had another night to savour.
The netminder made a fine save on 66 minutes when he kept out a Peter White shot before the inspired keeper made another wonderful save two minutes later from Ben Ryan.
After the visitors dealt brilliantly with the resulting corner, Blues defender John Frost slipped on the halfway line with Rafter taking full advantage when he raced clear on goal before firing to the corner of the net past the advancing Holden.
Rafter was denied his hat-trick by Blues keeper Packie Holden six minutes from time when his close range was kept out before Mark Griffin drilled the rebound wide of the far post, while Maguire had an effort ruled out for offside before Nathan Murphy struck the post in the final seconds.
WATERFORD UNITED: Holden; Mernagh, Breen, Long, Frost; Ryan (McGrath [73), White (Coad [75), Dempsey, Phelan (Doherty [56); Maguire, Walshe.
DUNDALK: Cherrie; Gaul, Whelan, Burns (O’Brien [75), Osbourne; Shannon, Shields, McDonnell (Thomas [88), Shanahan (Murphy [60); Rafter,
Referee: Pádraig Sutton (Clare)