Three tries inside the opening 37 minutes laid the foundations for this Vodafone Ireland WXV Series victory, but Japan made the hosts work for the result right until the very end.
With a strong wind at Ireland's backs, fit-again winger Béibhinn Parsons delighted the Dexcom Stadium crowd with a lightning-quick try from halfway, adding to a Brittany Hogan double.
The match took on a different complexion when the Sakura Fifteen capitalised on Erin King's yellow card with Moe Nagaoka driving over twice, either side of half-time.
Ireland stayed in control of the scoreboard, though, as Aoife Dalton's charge-down effort, coupled with Parsons completing her brace, saw them emerge as five tries-to-four winners.
Giving his reaction afterwards, head coach Scott Bemand said: "It was a tough game, and the Japanese are a good team. A well-organised team.
"We played them recently in the World Cup (last year) and knew that they would bring layers to their game.
"They have got a new head coach (Toshizumi Kitagawa), a second row, like he's going to be passionate about the lineout and the maul, and we knew that challenge was coming.
"So, we had some things to put right from last week, and actually in a tough game, I'm delighted that the team showed a bit of character, a bit of resilience, and the wherewithal to come through it."
Ireland had a strong set-piece platform to rely on throughout, picking up five scrum penalties and winning all bar one of their ten lineout throws, while pressurising Japan into overthrows.
In addition, the tight five contributed handsomely in the loose, with Niamh O'Dowd leading the tackle count with 20, a figure matched by Dorothy Wall in terms of carries. Tighthead Eilís Cahill also got through a lot of good work on her first start.
Following up on her debut last week, Jemima Adams Verling made the most metres of the forwards with 56.4 metres from seven carries. She was one of four starters from Connacht.
Another player to stand out at their home provincial ground was Anna McGann. It was her first international start in the outside centre position, and just her eighth overall in 19 Test appearances.
Dalton has been very difficult to dislodge from the starting line-up, but McGann, an athletic and rangy runner, proved a more than able deputy, supplying the assists for both of Parsons' tries and reaching double figures for carries (12) and tackles (13).
The Athlone native got game-time in midfield alongside both Eve Higgins, who came off in the 45th minute, and try-scoring replacement Dalton. Six of her previous starts were on the wing.
Reflecting on how his charges were able to problem-solve on the pitch to a greater degree this week, Bemand acknowledged: "Learning your lessons is always much better from a winning position, isn't it?
"Our execution, we thought we left a few points out in the first half on the tryline - two held up, one pushed into touch.
"And I think the scoreboard (19-5) should have looked a bit different at half-time, but we knew what we were turning around into.
"We love coming here to Galway, but there is a Galway breeze, and we knew that turning round into that, in some ways, it's a bit easier to play into the wind.
"But you've got to get through those moments, those possessions. The momentum, there was a balance, and a tipping point into it.
"I thought Dannah O'Brien was incredibly calm actually at outside half, trying to do the work through the middle, get the ball into Vicky Elmes Kinlan's hands, or Jemima's, and they're making metres out on the edge.
"Actually, they took a lot of pressure (away from) what could have been playing in and around our 22, which I thought we got that right, in the main."
He continued: "Sometimes you can win a game with the ball and sometimes you get some defensive pressure, and you know, seeing Béibhinn pick a ball that the line speed has created a bit of pressure, and Japan have put it down.
"Anna picks it up and Béibhinn streaks into the distance. Sometimes your defence has got to earn it as well. And when a strong wind is into you, your defence has got to do as much as your attack in terms of try-scoring.
"I'm delighted on the back of last week where we felt we gave entries too easily, conceded scores too easily, that the defence was able to create some pressure.
"There's still bits to go at. We're not the perfect article by any stretch of the imagination, but I'd say we'll take our lessons from a winning position this week."
It was an improved Irish defensive effort all-round, as the Munster trio of Beth Buttimer, a debutant on the day, Jane Clohessy, and Grace Moore came off the bench to contribute to a tackle success rate of 92.8%.
Moore won one of the girls in green's four turnovers, and lively loosehead O'Dowd chipped in with two dominant tackles. Still, the penalty count went against them for the second week running, finishing 16-11 in Japan's favour.
Buy Your Tickets For Ireland v Japan In Cork
The two teams will lock horns again at Virgin Media Park next Saturday, in a game outside the WXV Global Series window. Further improvements will be sought before the two-Test tour to South Africa.
Already looking forward to the Cork rematch with Japan, Bemand admitted: "It feels like a series. We get to back up. We were talking about it this week, and I've just called it part one. We want to back it up.
"It is really important because any of the learnings that we take from this week, it is who implements them faster, better.
"We'll keep going after us. There is a game out there that we believe, you know, it's that we don't want it to be a close game. We want to take the scoreboard away.
"There's some bits that we need to go after, absolutely. I think we're a step ahead than we were last week, but there's still a bit to go.
"Roll our sleeves up and we go again. We get a six-day turnaround, which feels a little bit different as well.
"We've been relatively used to seven days of late, so we'll look at getting what we need to on the training paddock, freshening up, and looking to attack down in Cork."
