The Australian forward, who stood out at the 2026 World Cup, arrives at Alvalade from Watford, with Sporting paying 15 million euros (£12.8m) for his transfer, plus another three million in add-ons, according to reports.
Irankunda has signed until 2031 and his deal is protected by an 80 million euro release clause.
He scored one goal in four matches for Australia at the World Cup. Last season, the 20-year-old winger played 42 games for Watford in the Championship, scoring four goals and providing four assists.
In his first words at his presentation, inspired by his celebration in tribute to Michael Jackson, Irankunda said he was "very happy and grateful to be" at Sporting.
"I accepted the offer thanks to conversations I had with people within the club's structure. 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. I'm ready," he told the club's media.
"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 this. When they told me about Sporting CP, it was Sporting CP all the way," he added.
Regarding his aims, Irankunda was clear: "I came to win titles and to play for great fans. I want to make history and there is no better place to do that than at Sporting. I am very happy to be here. I hope to win trophies this season."
Asked to describe himself, Sporting's newest starlet said he is "powerful, very fast and capable of jumping very high".
He added that he "can also score and create scoring opportunities for the team" and has no problem playing "anywhere in the attack".