Over 20 years after he made his Rovers debut, Terence Shiels' winning goal put his team into the second round of the FAI Senior Cup.
LETTERKENNY ROVERS 2
The match winner came on 87 minutes with Shiels drilling a free kick through the wall and in under the goalkeeper, Marcus Fitzmaurice.
Coming at such a late stage in the contest, there was to be no way back for the Dublin club who had played well over the 90 minutes.
Indeed they had big claims for a penalty when Graham Rusk appeared to be fouled by Shane Bradley on the hour.
They weren't happy either at the awarding of the free kick to Rovers from which 36- year-old Shiels delivered the winning strike.
“It was a very close game,” Shiels said afterwards.
“It just shows you, we can play football when we want, but we can battle too when we have to. And that was a battle out there.
“We gave away a soft goal for the equaliser and I was probably at fault.
“But we battled and got the victory and we're delighted to be through to the next round.”
Shiels could have done better in the lead-up to the Portmarnock equaliser which came after a rip-roaring opening to the tie.
He was robbed of possession inside his own half and when the ball was quickly fed in to Graham Rusk, the striker was brought down under the challenge of Rovers keeper, Rory Kelly.
Ross Moran converted from t the spot to make it 1-1.
The home side had gone in front on 9 minutes when Christy Connaghan raced onto David Shovlin's clearance and produced a wonderful finish to steer the ball over Fitzmaurice.
Connaghan was almost in again soon after but Darren McElwaine's through ball had just to much pace for the run of the midfielder.
Portmarnock however, had their moments too in a very open first half. Rusk saw a close range shot brilliantly saved by Kelly and the lively Alan O'Shaughnessy came in off the left only to see his well hit volley drift just wide.
The sides had drawn 1-1 in Dublin two weeks earlier and Sunday's replay at Leckview Park was another closely fought contest.
The chances weren't as plentiful in the second half and if anything, the teams seemed to tire. Both these clubs had been involved in FAI Intermediate Cup semi-final replays a week earlier.
But after Rusk's big shout for a second penalty, it was Rovers who carried the greater threat in the closing quarter.
Paul McVeigh caused more than a few problems on the left and it was from his pull-back that Connaghan almost scored a second, his well struck effort flashing just wide.
McVeigh was involved again soon after when he slipped McElwaine into the area and the striker's dipping shot clipped the top of the bar.
But with the game heading to extra time, Rovers snatched what proved the winning goal when Shiels' free-kick found a gap in the wall and flew in past the keeper.
Letterkenny Rovers: Rory Kelly, David Shovlin, Terence Shiels, Shane Bradley, Paul Boyle, Kevin McGrath (Brian McVeigh 88), Paul McVeigh, Christy Connaghan, Darren McElwaine, Mark Forker, Steve Okakpu (Tristan Ferris 62).
Portmarnock: Marcus Fitzmaurice, Aaron Egan, Nially Kenny, Callum Riches (Sean Moore 35), Gareth Whelan, Noel McGinnity, Ross Moran, Philip Rusk, Christopher Burgess, Graham Rusk, Alan O'Shaughnessy.
Referee: Michael Connolly.
Assistants: Marty McGarrigle, Paul Duddy.