Larne booked their place in Europe thanks to an incredible display from super-sub Ronan Hale.
The Invermen came from 2-0 down to beat 10-man Glentoran 4-2 after extra-time at the Oval on Friday night.
Hale was the hero, scoring all four of Larne's goals as Tiernan Lynch's side secured a lucrative spot in the Europa Conference League next season.
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Hale had two penalties on the night - his first was saved but he poked home the rebound.
He dispatched the second with aplomb and then completed his hat-trick in extra-time to put Larne in front for the first time in the game.
But the headlines would be reserved for his fourth, the 23-year-scoring from just outside his own penalty box after noticing Glens keeper Aaron McCarey was out of his goal.
McCarey had pushed up front as the Glens pressed for a last-gasp equaliser, but the attack broke down and Hale drilled a goalbound effort that bounced into the empty net.
It was an unbelievable goal to complete a remarkable comeback win for Larne.
For the Glens it was an agonising play-off defeat after they appeared to be in control of the game.
They led 2-0 through goals from Conor McMenamin in the ninth minute and Jay Donnelly just after the break.
But Paddy McClean was sent off in the 74th minute after pulling back Davy McDaid which resulted in Larne's first penalty.
Hale levelled from the spot in the 83rd minute and then forged Larne ahead in the 10th minute of extra-time.
And in the first minute of injury time at the end of the game, Hale delivered his wonder goal.
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