After recovering from a knee injury which sidelined him for the end of Munster’s season, Beirne made a significant impact off the bench in last week’s 33-31 win over Australia.
His inclusion in the second row is one of nine changes made by head coach Andy Farrell for the Japan game.
And while Beirne says he was buoyed by Ireland’s resilience, as they recovered from 12 points down to edge out the Wallabies, he admits his side must improve defensively.
“There’s a lot of positives, obviously,” he said.
“The fight from everyone to stay in it and come from behind was really pleasing. But I think there’s a lot of areas of the game where we probably weren’t particularly happy with the way we did things.
“Our defence wasn’t where it needed to be physically, and credit to the Aussies – they played incredibly well.”
Japan’s only victory over Ireland in 11 meetings came at the 2019 World Cup, but Eddie Jones’ side approach Saturday’s fixture in fine form after opening the Nations Championship with an impressive 27-10 victory over Italy in Tokyo.
“They play fast rugby. I’ve played them in the past they they like to keep the tempo of the game up – it’s on us to match that,” observed Beirne.
“But especially physically, I think that’s going to be a big area of the game where we’re going to have to put a stamp on it.”
