Cian Healy has backed Ireland to keep breaching the mental "dark spells" that can yield more World Cup progress.

James Ryan, Rory Best, Tadhg Furlong and Andrew Conway all crossed as Ireland blitzed Scotland 27-3 for a bonus-point victory in Yokohama on Sunday.

Ireland squeezed the Scots up front then ran them ragged out wide to provide the perfect start to Pool A.

Now Joe Schmidt's men will roll on to face hosts Japan in Shizuoka on Saturday, with prop Healy expecting a frenetic pace against the Brave Blossoms.

Leinster star Healy believes Ireland's relentless physical preparation has left the squad mentally robust enough to push into ever-increasing limits.

Asked how challenging the humidity had been against Scotland, Healy said: "It felt tough. It was hard to breathe, but we just have to back all the work that they have done with us, and go through the dark spots where you're sucking air.

"It was good, we played the game in the right positions and in the right parts of the field for the most part, backed our fitness, came through the dark spells and that's when we actually got to play a bit of our game and that's when we went harder into that zone.

"It just takes your breath in a different way, it's not like playing in the sun or anything like that.

"Your breath goes, but you can get it back quickly, if there's a break in play you can get regular pretty quick.

"But those extended periods of play do put a lot of pressure on the lungs, and we've done a lot of extended periods of training.

"So it's in the head then, it's not a shock, we've been that to place already so it's just a continuous reminder that it's all good, just keep going through it and get to those spots."

Japan thumped Russia 30-10 in Tokyo on Friday night, to open the first World Cup in Asia in style.

What Jamie Joseph's men might lack for bulk up front they always bid to mitigate by attacking with width and unrelenting pace.

And Healy insisted Ireland will continue to crack the whip in training, expecting a stern test at the weekend.

"I was lucky enough to be on the tour two years ago and that was incredibly tough," said Healy.

"They play such a high-tempo game, they've got a lot of great ball-players and great finishers. So it's going to be a tough preparation for it.

"We're going to be pushed for speed work in training, it's something we've been working on anyway, but definitely something we'll have to nail down to handle that attack of theirs."

Ireland could well look to rotate resources despite remaining wary of the Japanese threat, especially the natural boost of hosting the tournament.

Healy insisted he would always rather play every minute of Test rugby going than take a rest – but after 92 caps in 10 years at the top, the 31-year-old did concede he will take a slightly more phlegmatic approach now should rotation come his way.

Asked if he is more reasonable about rotation these days, Healy said: "No, but I'm understanding. I want to be involved in as many Test matches as possible.

"Whether it's starting or being on the bench I want to be involved and play games.

"But if the lads see that as the best option for us going forward, then now I'd probably have a more reasonable head to understand that decision."