Eight of Gatland's matchday 22 for the quarter-final showdown are aged 23 or under, which confirms the impact Wales' gifted new generation have made on the tournament.
There are no places in the starting line-up for British and Irish Lions quartet James Hook, Lee Byrne, Stephen Jones and Ryan Jones, with full-back Byrne and 102 times-capped Stephen Jones missing out completely.
Flanker Dan Lydiate's return from an ankle ligament injury means Gatland can reunite an outstanding back-row of Lydiate, skipper Sam Warburton and number eight Toby Faletau, whose combined age is just 66.
There is an opportunity in the full-back position for 22-year-old Leigh Halfpenny, while the bench features scrum-half Lloyd Williams, 21, and centre Scott Williams, 20.
"What has been great about this World Cup is that we've got a lot of youngsters with no fear factor," Gatland said. "They have got no history as well.
"We have just encouraged them to make the right decisions.
"We've talked about times when you want to be smart, play territory and the weather conditions, but if you feel there is an opportunity to move and play, you want to encourage that."
Gatland admitted his team selection meeting was one of the longest since he took charge almost four years ago as the coaches pored over every option presented by a fully-fit squad.
He added: "We didn't have a huge playing base in Wales and sometimes in the past if you have had three or four key injuries it has not always been about who you are going to leave out, but who you are going to pick in certain positions.
"The nice thing is I feel we have a balance we haven't had before.
"We've got players who can get us across the gain line, we've got pace out wide, we've got pretty special youngsters, we've got loose forwards that can carry, loose forwards that can compete on the ball, a couple of big second-rows and a more experienced front-row as well.
"We feel the balance of this team is the best we have ever had.
"What has been great is some of the older players know they've got youngsters on their heels that are really biting at their ankles. That has brought the best out of some of our older, more experienced players.
"It's a great position to be in to be having to make hard calls about who you leave out."
Despite Hook regaining fitness after a shoulder injury that meant he missed the final two Pool D games, in-form Halfpenny won Gatland's vote.
Hook, who has played Test rugby at full-back, centre and fly-half, has to be content with a place on the bench.
Halfpenny replaced Hook at half-time during the crucial 17-10 victory over Samoa in Hamilton 18 days ago, creating a match-winning try after he launched an attack from inside his own half. He then played on the wing against Namibia and Fiji.
Lydiate, meanwhile, returns to the starting XV after recovering from an injury that almost prompted a crushing early exit from the tournament, and Shane Williams' thigh strain has also healed, enabling him to wear his familiar number 11 shirt.
Lydiate's determined recovery process included him getting up every two hours during the night and icing an injury that initially threatened to destroy his World Cup dream.
"There are a lot of youngsters here who epitomise what you are looking for in a professional sportsman, the attitude, the way they handle themselves both on and off the field," Gatland said.
"The initial prognosis on Dan wasn't positive at all. It was very likely we were going to give him 48 hours and then probably have to send him home.
"I've already commented on the fact that he didn't sleep for 72 hours, icing his leg every two hours.
"It has shown to us what a great kid he is and what a fantastic professional he is in terms of being able to prepare himself and getting back available for selection."
Wales and Ireland recorded a victory each from their two previous World Cup meetings, but the last game was 16 years ago in Johannesburg and both teams boast peak form ahead of a semi-final clash against England or France next week.
There are mouthwatering individual battles littered throughout the contest - Warburton versus Ireland openside Sean O'Brien, Rhys Priestland against rival fly-half Ronan O'Gara, wings Williams and Tommy Bowe in opposition, to name just three.
And it has the potential to be the competition's best game so far as Ireland target a first World Cup semi-final appearance and Wales bid to match their achievement of 1987.
"It is the incentive that whoever wins on Saturday gets to stay here until the end of the World Cup," former Ireland coach Gatland added.
"You lose on Saturday and you are flying out on Monday. I can't see any greater motivation than wanting to perform on Saturday.
"Ireland had a lot of criticism pre-World Cup in terms of their warm-up games, but they played France twice and England as well.
"Those warm-up games, they count, but it doesn't matter if you lose, it's about fronting up and performing when it matters at the World Cup.
"Their first game against the USA wasn't the greatest performance, but then they had a great win over Australia and in the second half against Italy I thought they were outstanding.
"I have been incredibly impressed with Ireland."
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