Longford Town 1-1 (p) Cabinteely
Stephen McGuinness was the Cabinteely hero with three penalties in the shootout to put them through to the semi-final play after the sides could not be separated at City Calling Stadium.
Skipper McGuinness saved from Jack Doherty, Aaron Dobbs and Paul O'Conor before Rob Manley rifled in the winner from 12 yards.
Cabinteely now meets Drogheda United with their first leg back at Stradbrook next Friday.
After a subdued opening, the game burst to life on six minutes when Longford wasted a glorious chance to take the lead.
Dylan Grimes breached the visitors' defense when threading Dobbs in goal to draw McGinness out to the edge of his area.
Dobbs found Grimes striking a return pass with his winger's shot brilliantly cleared off the line by well-placed Eoin Massey with McGuinness.
That tenacious defending was absent at the other end as Cabinteely snatched the lead on 18 minutes.
Initially, goalkeeper Lee Steacy did exceptionally well to save Manley and then Marty Waters in turn.
But his defenders were slow to react to the loose ball as Paul Fox arrived to drill it into the net from close range for his third goal of the season.
Steacy was called into action again four minutes later, diving to his left to save Manley who got his head in for a deep free kick from Conor Keeley.
Longford pressed hard as time ticked towards the interval, getting their reward with the equalizer from another successive corner on 41 minutes.
Doherty put on a superb forced delivery with his left foot that was met by Dobbs whose flick header flew past McGuinness to the net, his fifth strike of the campaign.
An incisive move five minutes into the second half should have seen Town take the lead.
Byrne interchanged passes with Dobbs to race through on goal. The finish lacked conviction, though, as his weak shot was smothered by the well-positioned McGuinness.
Town were then almost caught napping from a 68th minute quick throw-in by Keith Dalton that found Fox not shooting too wide across the goal from a tight angle.
Five minutes later, home defender Conor Kenna had to virtually clear the line from Waters as Town failed to initially clear Dalton's free kick.
Back at the other end, McGuinness excelled with the game's save on 77 minutes, diving to his left to push Sam Verdon's drive round one post.
That was the last action of the 90 minutes with Cabinteely starting the first extra-time period on the front foot with both Waters and Keeley coming close.
Dobbs had Town's best chance at extra-time, heading straight at McGuinness from a Doherty free kick before Cabinteely skipper took the glory
Longford Town: Lee Steacy; Shane Elworthy (Joe Manley 105), Conor Kenna, Mick McDonnell, Dean Zambra; Jack Doherty, Paul O'Conor; Dylan Grimes (Peter Hopkins 76), Dean Byrne (Sam Verdon 72), Adam Evans (Aodh Dervin 95); Aaron Dobbs.
Cabinteely: Stephen McGuinness; Eoin Massey, Jack Tuite, Conor Keeley, Jack Hudson; Keith Dalton (Yuta Sasaki 87), Karl Manahan (Daniel Blackbyrne 62), Zak O'Neill (Shane Barnes 109), Paul Fox; Rob Manley, Marty Waters (Luke Clucas 113).
Referee: Alan Patchell (Dublin)