Ireland showed a huge amount of character and resilience to prevail in nail-biting fashion at Sydney's Allianz Stadium, with Ben Donaldson missing a last-gasp penalty for Australia.
Andy Farrell's men trailed by 12 points during the first half, as the Wallabies capitalised on an impressive start to register tries from Dylan Pietsch, Jock Campbell, Josh Canham, and Ryan Lonergan.
While being outplayed at times, and having defensive and set-piece issues to iron out, Ireland scored through Cian Prendergast and Josh van der Flier, and went in at the break boosted by an excellent Jamison Gibson-Park try.
Now just 24-19 in arrears, they hit the front when the excellent Hugo Keenan struck in the 46th minute, only for Tate McDermott to snipe over soon after for a 31-26 scoreline.
Captain Dan Sheehan had a try disallowed for an obstruction call against James Ryan, yet Ireland came through an attritional period - losing Jeremy Loughman to injury - to remain just five points adrift.
Too many team penalties saw Australian replacement Lachie Shaw sin-binned late on, and Thomas Clarkson duly barged over from close range to make it five tries apiece.
Sam Prendergast stepped up to coolly convert to the left of the posts, his kick proving to be the match winner whereas Donaldson drew two penalty attempts wide during the closing stages.
It will ultimately be termed an ugly win for Farrell's side, but hugely significant when you consider that Ireland had only beaten the Wallabies on five previous occasions on Australian soil - in 1967, and twice in both 1979 and 2018.
Minus some key men due to injury, including Robert Baloucoune as a late withdrawal, they showed their doggedness to pull off an important result for their Nations Championship hopes, and have banked a winning experience at one of their Rugby World Cup venues next year.
Addressing the vocal Irish support afterwards at a sold-out Allianz Stadium, Sheehan said: "Firstly I'd like to thank all the Irish who showed up, wearing green. We were really proud to play in front of you guys.
"I know you guys have had some late nights watching us, so thanks to everyone for showing up.
"To the Wallabies, what a Test match. I know you guys are proper going places. You can see it every game you play that you're going places.
"Thanks for the tough game. On another day that could easily have been yours, so fair play."
The dramatic outcome means that Ireland's longest winning streak against Australia now stands at six matches, going back to the 2018 series win, and they have retained the Lansdowne Cup as well as putting their first five Nations Championship points on the board.
They were very hard earned, especially when you reflect on the fact that the tourists missed 31 tackles and leaked 5.2 points per 22-metre entry. They also lost four lineouts and gave up two scrum penalties.
That leaves them with a number of things to tidy up for next Saturday's date with Japan at Newcastle's McDonald Jones Stadium. The Cherry Blossoms began their campaign with a 27-10 defeat of Italy.
Sheehan added: "Yeah, that test will stand to us. Not perfect by any means, but to be able to close out games like that, you know, you can't buy them.
"We'll be proud of that win. We've a lot to work on for next week and the week after, but we'll be proud of that."
Meanwhile, Keenan was hugely influential in his first appearance for Ireland since the 2025 Guinness Men's Six Nations, getting on the scoresheet and making some big defensive plays from full-back.
Returning to full fitness in recent months, he was back with a bang on the international stage, racing through for a well-taken try as well as brilliantly denying Joseph-Aukuso Sua'ali'i at the other end.
With Baloucoune adding to the back-three absentees, and Jamie Osborne playing on the left wing for the first time in green, full-back Keenan's presence was even more important, and he also had a stint on the wing during the final quarter.
Giving his reaction in the aftermath, the player-of-the-match said: "God, it was so tight, so intense. A proper Test match.
"But to be fair to the lads, we asked to make this crowd, these thousands in green, proud and I think that's exactly what we did.
"We never gave up, we dug in deep, and at the end of the day, it just came down to a kick, so hard luck to Australia. They were very unlucky, it could have gone either way."
Of course it was only last summer that Keenan battled through a severe stomach bug, and a longstanding hip injury, to emerge as the British & Irish Lions' try-scoring hero in the Test series Down Under.
His unforgettable 79th-minute effort in Melbourne confirmed the Lions as series winners, and he knew Australia would be keen to avenge that result, and also last November's 46-19 loss to Ireland in Dublin.
"They've got some unbelievable ball carriers, some unbelievable threats at the breakdown, and they're a top quality side.
"We obviously have come up against them a few times between autumn and the Lions. We know what they can be like, and how good they are," he added.
