Paddy Kavanagh's first half strike was enough to secure Shelbourne a 1-0 win against a Dundalk side who are now eight points adrift from safety in the Airtricity League Premier Division with just six games to play.
Kavanagh stepped up in the 40th minute to drive a fine right-footed effort past Peter Cherrie to give Alan Mathews' side the victory.
Fellow cup semi-finalists Dundalk went close on a handful of occasions through Marc Griffin and Michael Rafter, but their hopes of a comeback evaporated when Griffin saw red late on.
Buoyed by their FAI Cup quarter-final victory in Dalymount Park a week earlier, Dundalk started on the front foot and Rafter saw his header cleared off the line from a Marc Griffin corner inside the first minute.
Former Lilywhite David Cassidy looked the most likely on the Shels team to unlock the home side's defence, and his 14th minute pass set Paul Byrne clear only for the linesman to flag for offside.
Byrne had earlier been left fuming when he was taken out by Liam Burns en route to the penalty area; the Dundalk centre-back slightly fortunate just to receive a yellow card from referee Graham Kelly.
The home side's best chance of the half arrived in the 21st minute. Griffin whipped a fantastic free-kick across the face of goal but Stephen Maher could not control his effort and the ball sailed high and wide.
Griffin has rediscovered his scoring touch in recent weeks for Dundalk and the striker drew a fine save out of Chris Bennion in the 39th minute with a curling free-kick from the left.
Sixty seconds later and the visitors were in front. Paul Byrne and Cassidy combined well and Paddy Kavanagh needed just two touches to control and rifle the ball past Peter Cherrie from just outside the penalty area.
Frustration spread amongst the Oriel Park faithful as Dundalk huffed and puffed in the second-half. Skipper Chris Shields did go close in the 54th minute, his goal-ward effort hooked away by Stephen Hurley. But apart from that Shels looked comfortable.
Griffin forced another save from Bennion in the 75th minute with a trademark free-kick and the former Dundalk keeper, who won promotion with the club in 2008, tipped a Rafter effort around the post in the 81stminute before
Dundalk's hopes of a comeback disappeared when Griffin was shown a straight red by referee Kelly after a challenge on Brian Shorthall in the 85th minute.
Dundalk: Peter Cherrie; John Mountney, Derek Foran, Liam Burns (Barry Conlon, 85 mins), Eoghan Osborne; Mark O'Brien (Chris Reilly, 72), Stephen Maher, Chris Shields, Stephen McDonnell (Lorcan Shannon, 60); Marc Griffin, Michael Rafter.
Shelbourne: Chris Bennion; Brian Shorthall, Andy Boyle, Stephen Paisley, Sean Byrne; Stephen Hurley, David Cassidy (Barry Clancy, 80 mins), Glen Cronin, Paddy Kavanagh (Conan Byrne, 86), Stephen Hurley, Kevin Dawson, Paul Byrne.
Referee: Graham Kelly (Cork).