Hansen, the maverick speedster made in Canberra, went over for his three tries within the first half-hour and the Wallabies simply never recovered as they succumbed six tries-to-three in the pelting rain of Lansdowne Road on Saturday night.
Following gloomy defeats to England and Italy, the reverse has left coach Joe Schmidt staring at four-straight northern hemisphere defeats, the first time since 1958 this will have befallen any Wallabies team unless they can conjure up something special in Paris against Six Nations champions France next weekend.
So Lansdowne Road's Aviva Stadium, scene of so many of Schmidt's finest moments when coach of Ireland, this time offered the rugby folk hero one of his most dispiriting nights in his farewell Dublin Test.
And at the heart of it was one of those talents that has somehow got away from Australian rugby, the 27-year-old one-time Brumbie Hansen, whose mum was born in Ireland.
Injured during the Lions tour so he couldn't play in the Tests, the winger, who had only just returned from a niggling long-term foot injury, was sharp as a tack as coach Andy Farrell asked him to deputise at fullback.
Ultimately, his 28-minute treble did the damage, but it was compounded by a terrible final 10 minutes of capitulation at the end when captain Caelan Doris, Ryan Baird and Robbie Henshaw all ploughed over for Irish scores.
The Wallabies by that point were looking a shattered, weary rabble, down to 14 men after a late yellow-red for Nick Frost.
Harry Wilson's side had first-half scores from Len Ikitau and Fraser McReight, and a 74th minute consolation try from Billy Pollard, but with their attack blunt and their defence eventually shredded, Ireland recorded their biggest win over Australia.
"I've had a bang average run with injuries and everything, and I missed being in this team," smiled man-of-the-match Hansen.
"There's been a lot of talk that I wasn't born here and didn't grow up here - but this feels like home.
Any time I get to play for Ireland, it is a privilege."
His first score after six minutes was inspired by Jamison Gibson-Park's quick tap-penalty with the Australian defence still dawdling absent-mindedly.
Slick handling ended with Hansen sliding gleefully over, and six minutes later, after Max Jorgensen's defensive knock-on near his own line, the hosts delivered a carbon copy.
Soon after, John Eales Medallist Ikitau brushed aside flyhalf Sam Prendergast's challenge to respond, but as the rain pelted down, Ireland came again.
Prendergast's cross-field kick found winger Tommy O'Brien, only for the ball to squirm away from him and for Hansen, in typically eccentric fashion wearing one white boot and one black, to scoop up the ball and scoot over for his hat-trick.
His scores meant, remarkably, that an Aussie-connected player had scored the last five Test tries against the Wallabies following those of Melburnian Monty Ioane and Louis Lynagh, son of World Cup winner Michael Lynagh, which won last week's Test for Italy.
McReight burrowed over to narrow the gap to 19-14 at the break, but it was all Ireland on the restart, winger Harry Potter holding up Dan Sheehan on the line with a try-saving effort while the hosts were also unlucky to have an O'Brien try ruled out for an earlier knock-on.
The Australians were stagnant in attack, offering only power up front but no imagination behind, while Ireland forged further ahead with a drop goal from the controlling Prendergast and a penalty from Jack Crowley.
When captain Doris went over with 10 minutes left and Frost got sent to the bin for a shoulder-hit on Thomas Clarkson, the game was effectively up, even once hooker Pollard muscled in from short range.
It was only the prelude to a storming Irish finish as they took advantage of their one-man advantage to finish with two more scores in the last three minutes.
