St Patrick's Athletic moved back to the top of the Premier Division table on goal difference thanks to an early Paul Byrne goal at Richmond Park.
On an night of mixed emotions, Pat's had David McAllister sent off on 37 minutes and saw captain Damian Lynch limp off with a hamstring injury.
Saints took the lead after just four minutes.
Ian Bermingham's cross was knocked down by Ryan Guy into the path of Derek Doyle, who saw his snap shot blocked by Dundalk goalkeeper Peter Cherrie.
The ball fell out to Byrne who curled a silky shot past the diving Cherrie and into the bottom left of the net.
Pat's fans were calling for a penalty on 25 minutes when Dundalk's Liam Burns appeared to handle the ball in the box.
Vinny Faherty played on and cracked a hard shot off the post but referee Declan Hanney awarded a free out for a push on Burns in the build up.
Pat's were forced into making a change on 34 minutes when Lynch picked up a hamstring injury.
He limped off and was replaced by Derek Pender.
Pat's were reduced to ten men on 37 minutes when McAllister was given a straight red card after getting into a tussle with Stephen Maher, which came after a tackle by Maher on the Pat's man had not been punished by Hanney.
McAllister was clearly incensed and annoyed at the decision and had to be restrained as he left the pitch.
A man down, Pat's boss Pete Mahon went with a 4-4-1 formation after the break with Paul Byrne as a lone striker.
This proved successful as Dundalk found it hard to break down a resilient Pat's midfield and defence.
Paul Byrne and Stuart Byrne had further chances for Pat's in the second half while Guy went close with a dipping shot on 70 minutes.
Cherrie then saved a Paul Byrne effort with his feet on 72 minutes at his left post after Byrne took the ball down on his chest following excellent build up play involving Guy and Gareth Coughlan.
Faz Kuduzovic slammed in a good shot for Dundalk on 75 minutes but sent his effort wide while Pat's goalkeeper Gary Rogers saved a decent Maher header on 78.