Substitute Dan Hawkins scored with his first touch to seal this big win for Finn Harps at Dalymount Park.
Following Monday's 3-3 draw with Derry City, it's a damaging defeat for Bohemians in their hopes of making Europe via the league as Liam Burt's late goal proved too late to overturn earlier scores from Ethan Boyle and Hawkins.
With Dundalk and Waterford both winning, it marked a vital victory for Harps to keep them five points clear of the relegation play-off place.
Though Burt and Georgie Kelly troubled Harps early on, the visitors, showing no tired legs following Tuesday's FAI Cup exit in extra-time at Dundalk, came to play with two up top.
The power and pace of Tunde Owolabi won the visitors a sixth minute corner with Bohemians skipper Keith Buckley having to clear Ryan Rainey’s goal bound header.
The main action remained at the other end, though, Kelly bouncing a header wide before a brave block by Jordan Mustoe denied Keith Ward, who should have done better.
Sluggish play from Bohemians gifted Harps an opening on the half hour. The relief was palpable when Ryan Rainey arrowed his shot into the side netting.
Donegal native Kelly again headed wide from an Andy Lyons cross before the best piece of play of the first half deserved to see Bohs in front on 36 minutes.
Buckley tenaciously knocked Owolabi off the ball to find Ward in space. The Gypsies' number 10 cut inside to advance and strike a rising drive that 40-year-old Ger Doherty brilliantly tipped over the top.
A nervous start to the second half by Bohs presented Harps with a big chance two minutes in, Barry McNamee teeing up Owolabi whose close range shot was deflected over the crossbar by the alert Ciaran Kelly.
And further poor defending by Bohemians was duly punished on 55 minutes as Harps took the lead.
Rainey collected a loose clearance to thread the ball to right-back Ethan Boyle whose shot took a deflection off Anto Breslin to find the bottom right hand corner of James Talbot’s net.
With Bohs now chasing the game hard, dogged defending from Harps denied them an equaliser four minutes later, Boyle somehow getting back to deflect Ross Tierney’s shot out for a corner.
A minute later, Owolabi burst forward on the counter-attack only to blaze wide with the goal at his mercy before Harps again availed of dreadful defending to seal their win on 81 minutes.
Breslin allowed Karl O’Sullivan to get in his cross to the back post where Hawkins, just on the pitch, got behind Rob Cornwall to stretch out a leg and direct the ball to the net.
Burt’s fine low strike to the corner of the net in the third minute of added time represented mere consolation for Bohemians.
Bohemians: James Talbot; Andy Lyons (Rory Feely 67), Rob Cornwall, Ciaran Kelly, Anto Breslin; Keith Buckley (Conro Levingston 67), Dawson Devoy; Keith Ward (Ali Coote 67), Ross Tierney (Cole Kiernan 84), Liam Burt; Georgie Kelly.
Finn Harps: Ger Doherty; Ethan Boyle, Shane McEleney, Dave Webster, Jordan Mustoe; Ryan Rainey (Doherty 90+3), Johnny Dunleavy, Ryan Connolly (Mark Coyle 80), Barry McNamee; Sean Boyd (Karl O’Sullivan 68), Tunde Owolabi (Dan Hawkins 80) .
Referee: Ray Matthews (Midlands).