Shaun Williams' 52nd-minute winner for Fingal left Dundalk without a win in eight attempts.
Despite some neat early passages of play, the Lilywhites failed to trouble home keeper Darren Quigley and had their own custodian Peter Cherrie to thank for keeping out a Ronan Finn strike in the 12th minute.
Kenny Browne passed up a solid chance for Fingal when he headed just over from an Ollie Cahill corner after 18 minutes.
Dundalk player-coach Wayne Hatswell produced a stunning free-kick from 40 yards in the 32nd minute that looked destined to be goal of the season until Quigley tipped over.
Dundalk wasted a glorious chance before the half-time break when Matthew Tipton volleyed wide from close range.
Fingal went ahead seven minutes into the second half when Cahill's right-wing free went over the head of Daniel Kearns and Williams dived in to head past Cherrie from eight yards.
Another stunning long-range free-kick almost embarrassed Quigley in the 64th minute.
He failed to hold on to a 35-yard drive, but Dundalk conceded a free from the rebound and the danger passed. And to cap a bad night, the Lilywhites' Tom Miller was dismissed for a second bookable offence in the 83rd minute.