Tonga are "not delusional" about the challenge they face in their opening Rugby World Cup match against the world's number one ranked side Ireland but they are capable of causing an upset, assistant coach Tyler Bleyendaal said on Friday.

The Tongans face a tough task to reach the knockout stages for the first time as not only the Irish await them in Pool B but also defending champions South Africa and unpredictable Scotland.

Tonga though have been bolstered by an influx of former New Zealand Test stars, including Charles Piutau and Malakai Fekitoa, making them a potentially tricky opponent for all three tier-one sides.

Bleyendaal said his side were relishing the chance to be the fly in the ointment in Nantes on Saturday.

"That's why we are here," he said. "We are here to fire some shots and we are not delusional about the challenge we face, but we are excited."

Bleyendaal, a New Zealander who like several of the Tongan set-up played in Ireland, said a betting man would punt on Ireland winning 99 times out of 100.

Tonga did record a famous win over eventual finalists France in their 2011 pool match and the Irish slipped to a surprise defeat at the hands of hosts Japan in the 2019 World Cup. 

"The Irish are a pretty well-oiled team and it is a big challenge," said 33-year-old Bleyendaal, who had to retire from rugby aged 30 due to a persistent neck injury.

"Given the amount of times we cross paths with tier-one sides out of 100 games, how many would we be expected to win?

"However, one win suffices for us."

Without giving away their gameplan, Bleyendaal said that to combat the Six Nations Grand Slam winners, the Tongans would stick with their traditional strengths such as physicality.

Wing Solomone Kata is appearing at a second World Cup for Tonga, although the previous one was in rugby league.

The 28-year-old said he found little difference between the atmosphere in the different squads for both codes.

"(There is) no difference in the culture as we all come from the island," he said.

"There is humour, heaps of fun. Perhaps the game of rugby union is more tactical and it does have more smiley players."

Around 15,000 Irish fans are expected to attend the match making the atmosphere a daunting one for the less experienced Tongan players.

Kata, though, said the players with previous experience for tier-one nations could help their teammates in that respect.

"Some of the boys have been there before and can pass on that experience," he said.

While his teammate Vaea Fifita admitted to being nervous, Kata shrugged off the pressure.

"On the eve and the morning of the match I like to take myself out of the game," he said.

"I love to go for a walk, have a coffee and play cards."

pi/gj