The former Cliftonville man sent the big Larne support into raptures by heading home a Ronan Hale cross 13 minutes from time.
It was a game that had everything – a clash of the League heavyweights.
Larne lifted the trophy for the first time in their history last season – and they’ve certainly got into the winning habit.
In an intriguing contest, it looked as though penalties would be required to separate the teams – but Cosgrove had other ideas.
Blues boss David Healy made three changes to the team which defeated Oxford Sunnyside in the Irish Cup at the weekend, drafting in skipper Jamie Mulgrew, Sam Roscoe and striker Christy Manzinga.
It was rip-roaring stuff right from the first whistle with Mulgrew blasting a 25-yard shot inches over the top. But their opponents retaliated with Mark Randall threading a clever pass into the path of Doherty, whose low drive was smartly saved by Chris Johns.
Lynch’s boys had a great chance to break the deadlock on nine minutes. Doherty was flattened by a late challenge from Jordan Stewart and, when the Larne midfielder whipped in the free kick, Herron’s header crashed back off the foot of the post.
Again, the action quickly switched to the other end with Manzinga showing great pace down the left before picking out Stewart, who forced Rohan Ferguson into his first save of the evening.
Larne thought they had broken the deadlock seconds later when Albert Watson converted a cross from Cosgrove, but the linesman flag was already in the air.
The Blues were next to threaten with Jimmy Callacher’s sweeping 40-yard pass sending Kirk Millar free on the right and, when he spotted Stewart free in the middle, his cross was brilliantly flicked clear by Watson.
Healy’s team went perilously close again nine minutes before the break when Mulgrew’s persistence on the left created a chance for Stewart whose audacious 25-yard lob just fizzed over the crossbar much to the relief of Ferguson.
It was the Inver men who roared from the traps after the restart. McDaid managed to wriggle free from the clutches of Callacher on the left and, when the pulled the ball back into the middle, Cosgrove let fly with a stunning left-footed shot that inched just wide.
Ronan Hale then burst into action, cutting in from the left before curling a beauty just past the post. Then, in their next attack, Randall played in Doherty, whose rasping low drive was brilliantly clutched by Johns at full stretch.
But it was the Blues who almost forged a breakthrough just on the hour. Manzinga cut in from the right to fire in a wicked shot that flashed right across the face of the goal.
Larne, neat and tidy on the ball, went looking for the killer strike and, with 20 minutes remaining, Hale tried his luck from distance only to see Johns brilliantly paw his well hit 30-yard drive over the top.
If that was close, Linfield’s Matthew Clarke hit an equally stunning volley from the edge of the box that dipped just over, with Ferguson struggling – it would have been goal of the season.
It was now box-to-box and Johns again earned his wages with a brilliant save at the back post to deny a full-blooded drive from Dean Jarvis.
But Larne got the vital breakthrough 13 minutes from time. Hale sent in a teasing cross from the left that was met by the head of Cosgrove, whose booming header ballooned the net, much to the delighted of their adoring fans.
Linfield went looking for an equaliser and almost got it when Callacher’s delightful looked to be going into the net only to be headed off the line big Cian Bolger.