Both played the final half-hour with 10 men after straight red cards for Gavin Brennan and Gareth Matthews, and a late Philip Hughes goal ensured an exciting finish.
Shels boss Alan Mathews left a number of frontline players on the bench with Sunday’s FAI Cup semi-final in mind, while top scorer O’Brien returned to the Drogs’ starting line-up from suspension.
In a low-key first 15 minutes Drogheda’s Ryan Brennan had one header wide and then had a shot that was almost deflected in by O’Brien. But Drogs keeper Gabriel Sava then fumbled a Conan Byrne effort and Philip Gorman latched onto the rebound before shooting narrowly wide.
It was a narrow escape for the Boynesiders and they grabbed the lead goal from their next attack on 20 minutes. O’Brien was pulled back by Glen Cronin as he tried to latch onto Peter Hynes’ head flick, and ‘Fabio’ picked himself up to drive the resulting penalty down the middle.
There was a bit more vigour about Shels on the restart. Tempers boiled over just before the hour when Shels’ Gareth Matthews and Drogheda’s Gavin Brennan were sent off following an altercation and Shels continued to dominate without testing Sava.
The Drogs should have put the result beyond doubt on 78 minutes when Paul Crowley was put clean through by Eric Foley, but the skipper’s attempted chip landed in the grateful arms of Shels keeper Chris Bennion.
Ryan Brennan seemed to have sealed the points five minutes from time with a shot that appeared to be deflected past Bennion by Lorcan Fitzgerald.
But on 89 minutes substitute Philip Hughes replied for Shels and Drogheda survived six minutes of added time.
DROGHEDA UNITED: Sava; Gannon, Byrne, Prendergast, Hand; R Brennan, Crowley, Foley; O’Brien, Hynes, G Brennan. Subs: Brady for Hynes (67m), Marks for Brady (82m).
SHELBOURNE: Bennion; Matthews, Shortall, Paisley, Fitzgerald; C Byrne, Hurley, Cronin, Clancy; P Byrne, Gorman. Subs: Ryan for Shortall (67min), Hughes for Clancy (71min), Dawson for Cronin (84min).
Referee: Neil Doyle.