Taking his impressive form with Connacht onto the international stage, Sean Jansen marked his first cap with a bonus point try as Ireland made it back-to-back bonus point victories in the Nations Championship.
Andy Farrell's much-changed side outscored Japan by five tries to two - the last one from Tom Stewart coming in the dying seconds - but failed to fire on all cylinders, with their lineout letting them down.
One of the biggest positives for Ireland on the night was the performance of number 8 Jansen, a very deserving recipient of the player-of-the-match accolade.
The New Zealand-born back rower, whose grandparents, Maura and James, came from Monasterevin and Belfast, respectively, made over 50 metres from twelve carries, beating five defenders, and weighed in with 13 tackles.
He showed how explosive he can be with ball in hand, barging over for a well-taken score in the 51st minute, having previously gone close when hauled down five metres short before half-time.
Giving his reaction after the game, Jansen said: "I'm just so privileged to be able to wear this jersey. What a win. It was a dream come true for the four of us to win our first caps.
"When we found out, when the team got selected, us boys were just so stoked. It was a dream come true."
The 27-year-old, who got 61 minutes under his belt, was the only debutant in Ireland's starting XV, while his provincial team-mates, Sam Illo and Billy Bohan, and Ulster's Bryn Ward made their bows as second-half replacements.
Japan led three times during the first half, with winger Taira Main's fourth-minute try added to by the boot of Takuro Matsunaga, and they were only 19-13 behind at the break.
Ireland shook off some early errors to cross through Ulster duo Nick Timoney and Tom O'Toole, the latter notching his first international try, and returning centre Robbie Henshaw powered over in the 35th minute.
There were glimpses of a fluid and ever-sharpening Irish attack, and while chances were missed through taking the wrong option or Japan defending smartly, they were able to stay ahead on the scoreboard.
That eager bench helped, solidifying the scrum and aiding an improve defensive display compared to last week against Australia with six turnovers won and just ten missed tackles.
Jansen and Hayate Era swapped tries during the third quarter, before a Harry Byrne penalty left two scores in it at 29-20, and then Stewart used a Nathan Doak pass to wrap up the scoring with 80 minutes on the clock.
"We knew Japan were going to be a strong side," admitted Jansen, with a nod to their strong start to the Nations Championship when they beat Italy 27-10 in Tokyo.
"They are a very fit team, and they were in there for the whole 80 minutes, so credit to them. Just happy to win the game, and on to next week."
