Neve Jones, who went on to score a hat-trick of tries, Aoife Doyle and new cap Aoife Dalton replied with scores, the latter one set up and converted by impressive out-half Dannah O’Brien on her debut.
With Ireland’s strong set-piece game showing the work done under specialist coaches John McKeeand Denis Fogarty, the maul propelled hooker Jones over on two more occasions as 19-15 quickly became 31-15.
Flying full-back Méabh Deely helped herself to a memorable debut try, before the Japanese briefly broke up the Irish momentum with an effort from close range by replacement Makoto Lavemai.
Nonetheless, with some determined performances off the bench, Ireland made sure to finish the first Test on their own terms and take their handsome try haul to nine.
18-year-old talent O’Brien finished with a dozen points from the tee, with replacements Chloe Pearse, Molly Scuffil-McCabe, who ran in a fine individual effort, and Emma Hooban all touching down during a late flurry.
Natasja Behan, Leahy Tarpey and Taryn Schutzler made it six debuts on a night that saw the Ireland Women score their most points and tries since beating Scotland 73-3 to win the 2015 Six Nations title.
Jones, who continued her excellent form from the Six Nations earlier this year, became only the fourth Irish woman to score a hat-trick of tries at Test level, after Lynne Cantwell, Alison Miller (3) and Sophie Spence.
The sides will meet again at Tokyo’s Chichibunomiya Rugby Stadium next Saturday (kick-off 7pm local time/11am Irish time), when McWilliams’ charges will aim to clinch the series against the Sakura Fifteen.
As good as Ireland’s scrum and lineout were during this opening game, they provided the launchpad for Japan’s whirlwind start. Lesley McKenzie’s side exploited defensive gaps and showed the benefit of their recent matches together.
A nice move off a scrum led to busy flanker Kyoko Hosokawa carrying up towards the posts, and quick recycling allowed blindside Suzuki to score from a ruck just under the Irish posts. Ayasa Otsuka converted.
Even better followed for the hosts when a sweeping move from a lineout, out to the left, saw young winger Imakugi released for the try-line and a chasing Grace Moore was unable to deny her.
Japan threatened a third try before Otsuka knocked over a simple penalty, but Ireland got a grip on proceedings towards the end of the first quarter as captain Nichola Fryday led by example.
Tullow starlet O’Brien launched a penalty into the Japanese 22 and the forwards took over, Jones connecting with Fryday at the lineout and the well-organised drive delivered a try for the Ballymena native.
The left-footed O’Brien converted for her first international points, before strong-running centre Enya Breen, who had arguably her best game ever in the green jersey, spearheaded a superb break for try number two.
She dummied, arrowed through a gap and then linked with the pacy Deely who continued the move and it was her well-timed pass out of a tackle that put winger Doyle over near the left corner.
Number 8 Hannah O’Connor missed a long range penalty on the half-hour mark as Ireland looked to draw level on the scoreboard, but they redoubled their efforts just before the break.
After pressurising Japan into a lineout error, direct carrying from Linda Djougang, combined with a Breen turnover and a subsequent penalty, landed Ireland back within scoring range.
The resulting lineout did not go to plan, but Breen, Katie O’Dwyer and Fryday all made metres before O’Brien’s well-timed pass put 19-year-old centre Dalton powering through a tackle to give her side a four-point lead.
Openside Edel McMahon forced a penalty at the breakdown just after the restart, and Ireland took just 69 seconds to kick off the second half’s scoring. Jones connected with Fryday in the air before the lineout drive pocketed five more points.
Lovely hands from the backs had Ireland building promisingly from halfway again, although a penalty against McMahon allowed Japan to counter sharply before another handling error let them down.
Japan’s indiscipline, on the back of a defence-splintering run from Breen, gave the Ireland pack a sniff of the try-line once more, and Jones gobbled up another five-pointer from the maul. O’Brien converted crisply for a 31-15 lead.
Christy Haney made an immediate impact off the bench, winning a scrum penalty and then helping Jones to crash over for her third score of the night.
The midfield axis then combined to set Deely free from just inside the Japanese half. Dalton’s decoy run interested defenders and Breen’s inviting pass put the Ballinasloe youngster in between two defenders and there was no catching her.
Japan pulled the deficit back to 38-22 early in the final quarter, lifting the tempo and using a quickly-taken penalty to send replacement Lavemai barging in under the posts.
Unfortunately, the freshly-introduced Nicole Cronin injured herself in attempting to stop her and had to go off after suffering a clash of heads with Fryday. The versatile Scuffil-McCabe shifted to scrum half.
Djougang and ever-influential lock Sam Monaghan both managed to charge down kicks, the latter one allowing the alert Pearse to dot down the loose ball with 68 minutes on the clock.
The tries kept coming, Scuffil-McCabe spotting her chance to break from a ruck on the Japanese 10-metre line and her strength and speed ensured the grounding was good. O’Brien’s conversion took Ireland to 50 points.
With Monaghan hobbling off, Dorothy Wall switched to the second row and Tarpey came in to pack down on the flank. A pinpoint cross-field kick from O’Brien was brilliantly gathered on the bounce by Behan to keep Ireland on the front foot.
Breen missed out on a try that her performance deserved, just losing control of the ball close to the line, but the forwards added the icing on the cake.
Moore, who used her athleticism well in the loose, Pearse and McMahon all had a cut via a close-in scrum before Hooban reached out to score with little over a minute remaining.
Despite the ball falling off the tee as the clock went into the red, O’Brien showed a coolness beyond her tender years by drop-kicking her sixth successful conversion through the posts.
Full-time – Japan 22 Ireland 57