An injury-time strike from Chris Rodgers eased Larne into the fourth round of the Intermediate Cup.
After going behind, Ciaran Murray took his tally to four in two games to put the Reds in front. Yet again poor defending allowed the visitors to draw level before Rodgers popped up with a cracking finish to decide the tie.
Guilluame Keke, who was restored to the starting line-up, should have justified his selection on 45 seconds when he rounded the keeper only to drag his shot wide of the target.
But Larne fell behind after 11 minutes. Barry Tracey was allowed to cross from the right and Eoin Boyle pounced to fire home.
Six minutes later the home side were back on level terms when Murray picked up the ball wide on the right and thundered a shot from a narrow angle high into the net.
Larne then wasted a great chance to go in front. Eoin Gillan sent Thomas Robinson clear but with Murray free to his right he went it alone sending his shot well wide.
Just before the break the Reds edged ahead. A free from Samuel McIlveen was driven into the danger area and there was Murray to turn the ball into the net.
Larne were then reduced to 10 men when Gillan received a second yellow for a clumsy challenge.
With 20 minutes left on the clock Larne shot themselves in the foot once again as Downpatrick levelled. A cross into the box wasn’t claimed by keeper Paddy Flood, the defence had chances to clear before Stephen Galbraith stepped up to hammer the ball home.
But with extra time looming, Rodgers raced forward. He exchanged passes with Kevin Bradley before sending an unstoppable shot high into the net.
Larne: Flood, Bradley, McGonnell, Rodgers, McMullan, E.Gillan, Irivne (Donnelly 70), McIlveen, Murray, Keke (Murray 59), Robinson (McGrandless 56). Subs not used: McClean, McCabe.
DOWNPATRICK: McKenna, Fitzsimons, Connolly, Walsh. Kearney, Sharvin, Tracey, Telford, Galbraith, Boyle, McGarry. Subs: Magee, Traynor, Holland, A.Telford.