Out-half Ross Byrne will need to impress if introduced off the bench against Tonga (live on RTÉ2 and RTÉ Player) as the battle to be Johnny Sexton's number two heats up.
Head coach Andy Farrell has opted for a formidable starting team for their second match in Pool B, with just four changes from the side that dismantled Romania last time out.
Captain Sexton keeps his place as the Leinster man returns to full match sharpness, with his performance in Bordeaux all the more impressive given it was his first competitive outing since March.
Injuries aside, the 38-year-old looks set to start all the pool games, so attention turns to who gets the nod in the match-day 23 with challenges against South Africa and Scotland looming.
Byrne should get his first taste of World Cup action in Nantes after being named among the replacements, while Jack Crowley did his own chances no harm against Romania.
Speaking on RTÉ Radio 1’s Morning Ireland, Donal Lenihan said the contest between Byrne and Crowley is an intriguing one.
"I think Jack Crowley has grown into that role (Sexton’s back-up) over the last 12 months," he said.
"I thought he was very good again when he came on against Romania last week, but Ross Byrne is very comfortable playing in the backline given the vast majority come from Leinster. He will get his opportunity off the bench.
"Sexton got 65 minutes last week. I think if he got 50 this week, and Ireland are in a good position, I’d imagine you’d see a change there.
"If you want to win a World Cup, you have got to have competition for every place. Ross Byrne has to grasp his chance tomorrow if he wants to be part of the matchday 23 from next week on with South Africa and Scotland to follow, and hopefully the knockout stages after that."
Lenihan expects a stern challenge from Saturday’s opponents.
Former Munster centre Malakai Fekitoa and ex-Ulster full-back Salesi (Charles) Piutau are both included in a star-studded line-up, the pair among four ex-All Blacks in the Tonga line-up, with Vaea Fifita and Augustine Pulu also included.
William Havili, younger brother of current All Black David Havili, starts at out-half with the experience of Pulu inside him.
"When you look at the team they announced yesterday, they are all playing with top quality clubs, either in the URC or France," Lenihan said.
"Bear in mind as well they didn’t play in the opening round of games, so they have been sitting back just waiting for a chance to take on the number one ranked side in the world.
"There is a reason why Andy Farrell has gone for a very strong team. You look at France struggling to put Uruguay away last night and failing to get the four-try bonus.
"The World Cup brings out different animals in players. I expect Ireland to prevail. With a bit of patience they will win, and win well.
"Most importantly, the team will bag some game time and continuity with this selection for the big game against South Africa next week."
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