Yokohama - World Rugby's No 1-ranked team Ireland are peaking at just the right time, assistant coach Andy Farrell said on Wednesday ahead of their Rugby World Cup opener against Scotland on the weekend.

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Such was the bite and desire at training, Farrell said, that the coaches didn't even need to be on the paddock in some sessions.

"This week, there's certainly a feel of something that's brewing that's of major importance to the squad," said Farrell, who will take over from Joe Schmidt after the Kiwi stands down as Ireland head coach after the World Cup.

"The intensity, the bite that's in training is very significant and there's a completely different feel, so I suppose that's a culmination of what's happened over the last couple of months into this week.

"The build-up, the week, the buy-in, the grabbing hold of the week by not just the senior players but everyone in the group has been very significant."

Farrell, a former Wigan and Britain rugby league legend who had a brief spell in union as a player during which time he won eight caps for England and appeared at the 2007 World Cup, said experience was showing through in a squad boasting almost 1 300 caps.

"We trained unbelievably well on Monday," he said. "As coaches we didn't need to be on the field because of the ownership.

"We have experienced players... and they know what it takes to build through this week.

"That's what you want, they're sorting themselves out and that's where we want to be.

"We are building because we don't want to reach boiling point on Wednesday or Friday."

Farrell, who was defence coach for England at the last World Cup when they became the first host nation to fail to progress from the pool phase, likened Gregor Townsend's Scotland side to a "box of tricks", saying they liked to take calculated risks while playing at a high tempo.

"We need to expect the unexpected and try to get ahead of the game.

"We need to make sure we create opportunities from our defence to combat that."

Farrell confirmed that centre Robbie Henshaw, who missed the opening two games of the 2015 World Cup, was out for the weekend, but added that he was expected to be back for the second pool game, against Japan.

"The progress has been great and he's hitting all the markers as far as a return to training etc so they're happy with how he's progressing and hopefully he'll keep that upward curve," he said.

Aside from hosts Japan, Pool A also includes Russia and Samoa.

Progression from the pool for the two top finishers would set up quarter-finals against the winners and runners-up from Pool B, likely to be a permutation of New Zealand and South Africa.

That means Farrell might only get to pitch himself against his son, current England playmaker Owen, from the semi-finals.