Ireland ended Australia's hopes of a first grand slam in 32 years after digging deep to to secure a 27-24 win in Dublin on Saturday.
Having already beaten Wales and Scotland, Michael Cheika's men were looking to continue their winning run against teams from the British Isles, with Eddie Jones' in-form England to come next weekend.
But Ireland showed stunning resolve to collect victory, following up their triumph over New Zealand at the start of the month.
In doing so they became only the second northern hemisphere side to beat South Africa, New Zealand and Australia in a calendar year - matching England's feats of 2002 and 2003.
Despite squandering a 17-0 lead to trail by four points with 15 minutes remaining, Joe Schmidt's men highlighted their battling spirit to grab a match-winning try though Keith Earls.
That thwarted a superb comeback from the visitors, who had shown an ability to edge tight matches when beating Scotland by a point and France by two in their most recent outings.
The result represented a fitting way for Rory Best to celebrate his 100th international cap, which was played out against the backdrop of a magnificent atmosphere at the Aviva Stadium.
Ireland – beaten 21-9 by an All Blacks side intent on revenge last time out – made an excellent start, but they were made to wait until the 18th minute before Paddy Jackson's penalty broke the deadlock.
With the hosts dominant in possession and territory, Iain Henderson's try – which followed the sin-binning of Dean Mumm – was just reward.
Jackson added the extras as Australia's 14 men were punished in the 24th minute and there was more to come from Schmidt's men.
Garry Ringrose did brilliantly to evade several tackles and touch down, leaving Jackson with a straightforward conversion for a 17-0 lead.
But the Wallabies finally stirred into life just before the interval, with Israel Folau handing off for Dane Haylett-Petty to run under the posts.
The margin was down to three early in the second half as the visitors showed quick hands to get Tevita Kuridrani in down the left wing for a classy try, which was nervelessly converted by Bernard Foley.
With their 17-point cushion having almost disappeared, Jackson's 50th-minute penalty was vital in steadying the ship.
However, after Folau selfishly failed to off-load when two team-mates were better placed out wide, Sefanaia Naivalu's converted try put Australia ahead for the first time.
Foley then kicked a 60th-minute penalty, but Ireland summoned something special to hit back with an excellent score from Earls following sustained pressure.
They had to ride out a tense final spell in which Australia desperately sought a route back into the match, but the home side held firm.