The talisman stepped up deep into stoppage time to break Glenavon hearts and secure a 2-1 win at Inver Park.
The game looked to be heading towards extra time after Davy McDaid had cancelled out Daniel Purkis' opener.
But Tiernan Lynch s men kept plugging away and grabbed the all-important winner to earn home advantage for the final.
After resting several key players for Saturday's 2-1 defeat at home to Coleraine Lynch recalled his experienced campaigners for the clash.
There were still without Conor Devlin though and both Graham Kelly and Jeff Hughes were also ruled out through injury.
Gary Hamilton only made one change to his Glenavon side with Calvin Douglas replacing Aaron Harmon.
Fresh from bagging a goal in the 1-1 draw against Dungannon Swifts at the weekend Daniel Purkis broke the deadlock at Inver Park inside five minutes.
The striker managed to head home from close range after the hosts failed to deal with a corner from the Lurgan Blues with the ball flying about the penalty area.
The lead only lasted three minutes though as McDaid tied things up.
He steered the ball home from a Ronan Hale effort after Mark Randall's header was cleared off the line from Donnelly's corner.
The striker almost put his side in front in bizarre circumstances on 13 minutes.
Visiting keeper Craig Hyland failed to see McDaid lurking behind him when he dropped the ball to clear, the front man nicked in but a combination of the goalkeeper and Douglas managed to deny him.
Both sides had the ball in the net before having them chalked off as McDaid was adjudged to have been offside while at the other end Matthew Fitzpatrick's goal was ruled out for a foul on Conor Mitchell.
As the half progressed Larne looked to be the more dangerous side, but Glenavon were matching them with their workrate.
Despite carving out the better chances it was the Lurgan Blues who nearly scored again before the hour mark as Fitzpatrick tested Mitchell from the edge of the box with the keeper doing well to tip over.
The hosts then had a big shout for a penalty just past the hour mark as McDaid went down under a challenge in the box, but referee Lee Tavinder was unmoved.
And there was appeals again soon after as a Donnelly free kick hit a hand and the hosts protesting it was inside the area, but a free kick was the result.
At the other end Campbell whistled an effort inches past the upright.
As we moved into the dying seconds Albert Watson headed narrowly wide.
But there was still time for some late drama at Inver Park.
In the third minute of additional time McKendry linked up with Jarvis before feeding Donnelly inside the box, and hr produced a typically composed finish to win the game for Larne.
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