A 93rd-minute winner from Chris Lyons sent United Park into delirium and gave Drogheda United the edge over Finn Harps with a Premier Division place in 2020 at stake.
He latched onto Stephen Meaney's backheel with just seconds remaining and produced a fine finish to hand Drogheda a slender lead ahead of Friday's second leg.
There was still time for substitute Raff Cretaro to hit the post at the other end as a largely lacklustre game sparked to life at the death.
Lyons had the outstanding chance of the first half.
Adam Wixted delivered a pinpoint cross from the right but despite being unchallenged in the area, Lyons header was too close to Harps goalkeeper Mark Anthony McGinley.
The ex-Bray striker tested McGinley again when he drove past Gareth Harkin and unleashed a dipping effort than the goalkeeper had to tip over the crossbar.
The opening 45 minutes was even on the whole but Lyons' header aside, neither side created much.
Harps hit the post when Ruairi Harkin's deflected cross came off the upright and Nathan Boyle's blushes were spared when the offside flag was raised as he skied over from two yards out.
The striker had an open goal at his mercy.
Sean Brennan went closest for Drogheda in the second half with a quick free-kick that almost caught out the visiting goalkeeper.
Quality chances, in truth, were at a premium.
Ollie Horgan was sent from the dugout by referee Rob Hennessy on the hour mark.
His prolonged protests after Harry Ascroft was not allowed back on the field after a change of his shorts were deemed enough to warrant his exit.
Sub Stephen Meaney had United's best second half opportunity within moments of his introduction but he shot over from 12 yards.
Raff Cretaro's introduction did breath some life into Harps but it wasn't enough.