Ireland must discover a response to the question that no one in this World Cup has yet come close to answering: how do you stop George North? The emergence of the flying teenager, who is built like a Maori but has the speed of a sprinter, mirrors that of Wales at the World Cup. Other than the All Blacks, these Celtic cousins have become the most talked-about teams in New Zealand.
The 19-year-old wing embodies the confidence and swagger of youthful exuberance at the heart of the Welsh revival. He has cut a swathe through defences thus far, scoring nine tries in 12 Tests, and is threatening to become a new global star of the game.
It is not even a year since he made his international debut and for such familiar foes, given the weekly rivalry in the Celtic league, North provides an element of the unknown.
The Ireland winger Andrew Trimble said: "There will have been few teams who knew anything about him [North] at the start of the World Cup but everyone will have seen his recent performances. He's a big man, fast and powerful, and has really gained confidence on the back of those displays.
"Usually against the bigger guys you just throw yourself in and see what happens. But he has such quick feet and strength that he can go around you just as well as through you. We'll be doing a lot of work this week with [defence coach] Les Kiss but I guess you have to stay on your feet and make sure you get your body position right."
Wales must choose two from North, Leigh Halfpenny and Shane Williams this weekend, though Ireland's Swansea-based Tommy Bowe knows who he would rather not face.
He said: "North has had an outstanding tournament already, but I've had trouble with Shane over the years and I know from working with him at the Ospreys that he's a guy I would prefer to have on my team than play against."
History beckons for both teams in Wellington, given Wales have not reached the semi-finals of the World Cup since here in New Zealand 24 years ago while Ireland, to their constant chagrin, have never reached the last four.
Ireland lock Donncha O'Callaghan said: "We would like to do something to separate us from Irish sides that have gone before. They have won a Grand Slam, Triple Crowns and reached World Cup quarter-finals, but this could be the chance for us to do something special and stand out on our own.
"People might talk about the last few years being a golden generation, but what have we done to separate us from the rest? Not a whole lot."
The last Ireland World Cup campaign was the country's worst after they failed to progress beyond the pool stage, and the giant second-row admits that pain still motivates the Irish today.
He said: "The driving force is the last World Cup. I still remember the sick feeling at being home so early with your head still spinning. I never want to experience that again and I'll do anything to avoid it."
The Irish and Welsh members of the 2009 Lions built a strong bond on tour in South Africa and one can only imagine the "craic" when O'Callaghan roomed with another "lively" character in Andy Powell.
It is no wonder O'Callaghan has a soft spot for Cardiff having made his Test debut in 2003, won two Heineken Cups and a Grand Slam in the Welsh capital. However friendship only stretches so far with a World Cup semi-final place at stake.
O'Callaghan said: "I always find the Welsh game the most competitive game because every ball is contested. Against other teams it's hugely physical, but you'll always be out on your feet after the Welsh game because it is played international pace for the whole 80 minutes. It is hugely intense."
The odds on Rory Best playing on Saturday were described by Ireland as "long" so Sean Cronin looks set to make a rare start, admitting the World Cup quarter-final was a different level altogether to what he has played before.
There were no such problems for Wales with James Hook, Shane Williams and Dan Lydiate all passed fit to finally resume training today.
Few could have imagined before the tournament began that, given the form of the Welsh back-line, the most important of that trio would be Lydiate. Given his destructive capabilities in the back-row contest that will go a long way to deciding the outcome, though, that is how it is. Jamie Roberts, Gethin Jenkins and captain Sam Warburton are also fit after niggles.
O'Callaghan said: "The breakdown is where the game is at right now and Warburton has been incredible. In this tournament you would put him up there with the likes of [Richie] McCaw and [David] Pocock when it comes to whipping your ball."