History beckons for one of these teams and for the loser, a long journey home to ponder what could have been.
Warren Gatland and Declan Kidney are both acutely aware that this presents their greatest opportunity to take their teams to the semi-finals of a World Cup and, with France or England awaiting them, possibly beyond.
"It's all about knock-out rugby," Gatland said. "The incentive is that whoever wins gets to stay until the end of the World Cup but if you lose, you're going home on Monday. There's no greater motivation than that."
Other than the All Blacks, these are the two most talked about teams in New Zealand, given Ireland's victory over Australia and their horde of fans, while the Welsh style of rugby has attracted many admirers.
Tomorrow, however, will bare witness to their contrasting ideologies.
Ireland have stuck to their tried and tested formula, with Ronan O'Gara expected to kick the leather off the ball given the wind and rain expected in Wellington's 'Cake Tin'.
"Ireland have a very experienced squad that have won a Grand Slam and Heineken Cups, but this is the World Cup," Gatland said.
"They had a lot of criticism before the World Cup but it doesn't matter if you lose warm-up games, it's about fronting-up in the tournament and that's what they have done."
Ireland's players have a track record for winning cup games at provincial level but have struggled to transfer that to international level.
Wales have little cup pedigree and have instead opted for the confidence of youth, having left out Stephen Jones, Lee Byrne and Ryan Jones for Rhys Priestland, Leigh Halfpenny and Dan Lydiate.
"Both teams know each other very well but the Irish won't know as much about some of our younger guys," Gatland said. "They have no fear of history and we've encouraged them to have a go. We have a balance that we've not had before."
The key to the result will surely lie in the back-row.
"Ireland have arguably the best back-row in the tournament," Lydiate said.
"(Sean) O'Brien is their go-to man, someone who trucks it up for them. It takes a lot of people to stop that but it's our job to try to nullify him. Sam (Warburton) is destructive at the breakdown and Toby (Faletau) is an awesome ball-carrier. It's going to be a great battle."
Brian O'Driscoll said: "You have to do what gets you across the line, what wins games. It's about playing pressure rugby, not necessarily deciding on one type of brand before the game. A big factor is going to be conditions and how much of an expansive game the weather allows you to play."