An Eamon Zayed goal four minutes into stoppage time at the end of this one saw defending champions Drogheda book their place in the last four of this year's Setanta Cup.
Behind to a first-half Derry goal and playing second fiddle to their opponents for the opening forty five minutes United were the better side in the second period, however, that late winner was harsh on Derry who deserved at least a share of the spoils.
Derry went in search of a goal right from outset and went very close to opening their account twice in quick succession.
Killian Brennan's cross from the left found Ruaidhri Higgins whose shot was punched out by keeper Mikko Vilmunen to Greg O'Halloran who saw his snap shot pushed out for a corner by Vilmunen.
Almost immediately Derry won a free on the right which Brennan floated into the box for Peter Hutton who glanced his header wide.
It was all of 25 minutes before Drogheda seriously threatened the Derry goal when Paul Keegan's quick free from the left picked out Eamon Zayed who saw his header fly narrowly wide.
Derry's Brennan again found space down the left after 38 minutes and his cross was met by Mark Farren who clipped his shot wide.
The visitors struck for a crucial lead goal right on the half time whistle when Higgins corner on the right was only cleared to the edge of the box where Kevin Deery was on hand to send his shot through a crowded goalmouth to the net.
Ten minutes into the second half Drogheda won a corner on the right which Paul Keegan swung over for Gavin Whelan to guide his header back across the box for Zayed who volleyed over from fifteen yards.
With 65 minutes on the clock Derry pushed down the left flank and Sean Hargan's cross fell to Mark Farren who lifted the ball over the bar from six yards.
Drogheda were back in the match in the 69th minute when Zayed pushed the ball out to Whelan on the right side of the Derry penalty area.
Whelan's shot was too hot for keeper Pat Jennings to handle as he spilled the ball to Tony Grant who was perfectly positioned to score.
Within a minute Derry almost regained the lead when Higgins free from twenty yards shaved the crossbar.
In the closing stages Grant was denied a second goal for United when he ghosted past two challenges before unleashing a shot which rebounded off Jennings legs to safety.
A late Derry flurry saw Vilmunen tip Deery's free to safety, Farren head narrowly wide and Higgins test the alert Drogheda net minder from 25 yards.
Just when it appeared the game would end in a draw a long ball towards the Derry box saw Jennings race from his line and attempt a headed clearance outside his area, however, the ball spun off his head and into the path of Zayed who guided his header to the empty net for the winner.