Sporting announced the move from English Championship club Watford on Saturday, with the undisclosed fee reported to be in the region of $26 million and potentially rising with add-ons to $29.5 million.
Ending a week in which teen defender Lucas Herrington became the most expensive Australian footballer of all as he joined Premier League Hull from MLS side Colorado Rapids for a reported package worth $33m, Irankunda's move was another red-letter day for Aussie soccer.
Sporting, the 21-time champions from the capital Lisbon, were certainly making a song and dance of the signing of the youngster who lit up the World Cup for the Socceroos as they revealed they've inserted a whopping $130 million release clause into Irankunda's contract.
They also greeted the 20-year-old Australian with one of the most innovative welcome videos seen in European football.
To the strains of Michael Jackson's 'Bad' amid 'Wanted' posters, Irankunda stepped out of a sports car and reprised his Jacko-themed goal celebration before metamorphosing into the famous green-and-white hoops.
Actually, 'not Bad'.
Sporting, who once nurtured the talent of another young winger called Cristiano Ronaldo, clearly think a lot of Irankunda, snapping him up in a five-year deal through to 2031 even though he's yet to truly establish himself at a European club, either in Germany or England.
Negotiations over the sale were complicated because Bayern Munich, who sold Irankunda to Watford, will also get a share of the sell-on. The German giants are expected to garner nearly $10m from the move.
Irankunda made no bones about his delight at joining a club that can offer him Champions League football this season, after an up-and-down spell at Watford where the highs were soaring with four spectacular goals.
"It's a moment of great happiness and pride," he told the club's fans. "I'm very excited.
"Any child or person who loves football dreams of playing in the UEFA Champions League. Having this opportunity will be fantastic and a dream come true.
"The project seemed very good to me and, obviously, Sporting CP is a top club with a great history. I wanted to accept this challenge and I'm ready.
"Even someone who lives on the other side of the world, like me, looks at Sporting CP and understands how fantastic and passionate the fans are.
"That's why, and also because of the club's history, I get goosebumps thinking that I'm wanted by a club like that.
"When they told me about Sporting CP, it was Sporting CP all the way.
"I came here to win titles and to play for great fans. I want to make history and there's no better place to do that than Sporting CP. I'm very happy to be here and I hope to win trophies this season."
Sporting, who nailed down their Champions League place by finishing second behind Porto last year, have already begun their season, picking up their opening Primeira Liga win on Friday with a 3-2 win over Vitoria to move top.
Australian players have been in demand since the Socceroos reached the last-32 at the World Cup.
BIG-NAME AUSTRALIANS ON THE MOVE SINCE THE WORLD CUP
Patrick Beach [Melbourne City (Aus) to Troyes (Fra)]
Cammy Devlin [Hearts (Sco) to Rangers (Sco)]
Ajdin Hrustic [Heracles Almelo (Ned) to Al Nasr (UAE)]
Jackson Irvine [St Pauli (Ger) to Cerezo Osaka (Jpn)]
Awer Mabil [CD Castellon (Esp) to Melbourne City (Aus)]
Paul Okon-Engstler [Sydney FC (Aus) to FC Koln (Ger)]
Lucas Herrington [Colorado Rapids (USA) to Hull City (Eng)]
Nestory Irankunda [Watford (Eng) to Sporting (Por)]
