Injury-hit Ireland must lay it all on the line and time for new breed of stars to step up to the plate in Wales showdown
SO MUCH rests on this game.
For the visitors, a season is there to be salvaged. Defeat in Twickenham has left Wales in the last chance saloon but with enough belief left to breed confidence that they can get their title tilt back on track.
Ireland? Well they would appear perfectly poised to keep their Grand Slam show on the road, but beneath the long unbeaten run since these teams met last season is a series of undermining factors, not least the ever-lengthening injury list.
The sight of Johnny Sexton receiving treatment on his lower back and missing the beginning of yesterday's captain's run at Lansdowne Road was a chilling one for fans of his side.
At his eve-of-match press conference, Rory Best allayed fears that the out-half was injured and it was just as well because the playmaker would be one loss too many.
Already operating without front-line players in every department except half-back, the absence of the leading out-half in the tournament would derail Ireland's prospects.
The decision to allow Ian Keatley run the bench for Munster last night suggests the No 10 will be fine.
Already without Tadhg Furlong, Iain Henderson, Jamie Heaslip, Sean O'Brien, Jared Payne and Robbie Henshaw, among others, the onus is on a new breed of Irish stars to step up to the plate in the biggest game of their careers to date.
Individually, Dan Leavy, Chris Farrell, Andrew Porter, James Ryan, Bundee Aki and Jacob Stockdale are talented players with the capacity to thrive in the Test environment, but thrown in together collectively there is a concern about Ireland's experience.
Alongside them is a bunch of hardened professionals with 577 caps between them. They will have a role in helping the new men through.
Depth has been a focus for Joe Schmidt ever since that fateful defeat to Argentina and this is the biggest test of the work he has done since.
The process began in the 2016 Six Nations in this very fixture and 10 of today's match-day 23 were introduced during the period since.
Fast-tracking a trio of stars from the U-20 team that reached the World Cup final that season is a clear pointer to the long-term focus, but one imagines Schmidt never envisaged Porter, Ryan and Stockdale starting a crunch Six Nations game together this season.
The loss of Furlong is big, but placed alongside the injuries to Henshaw and Henderson it is seismic.
Henderson has arguably been Ireland's best player in the campaign to date and along with Furlong he was taking on a huge amount of the responsibility for Ireland's attacking game-plan while also helping to lead the defensive line.
Henshaw was adapting to the No 13 role, but offered so much to the team in terms of defensive leadership and attacking thrust.
Despite his youth, his international experience was key to helping Aki and Stockdale to adapt.
The young winger is a particular worry and Wales will have noted his defensive struggles with interest.
Playing outside Farrell, who has been good for Munster and offers plenty of physicality, the Ulster youngster needs an error-free afternoon.
Rob Kearney will be key to keeping him honest positionally, while Sexton and Conor Murray will do their fair share of defensive work too, but there will be moments when Stockdale will be presented with defensive decisions and the responsibility will be his alone.
Going forward, he has been excellent and he adds a layer of danger to Ireland's attack, but questions linger over his defence.
Up front, Porter makes just his sixth senior start on the tighthead side of the scrum.
He is a physical specimen, long-touted to step up to this level and he will have a long career dove-tailing with Furlong if he can stay injury-free, but even the Wexford man struggled during his first season as an international.
The Welsh scrum may not have the formidable reputation of some others, but the all-Scarlets front-row got the better of England two weeks ago and will relish the prospect of taking on a relative greenhorn.
If Porter survives and goes on to thrive, it will be a significant battle won for Ireland.
Behind him, Ryan will bring his customary aggression to the party while Leavy must start to dominate internationals the way he does club games.
He'll rate himself as good enough to do it and if Ireland can win the breakdown then they'll be on their way to victory.
Breaking Wales down has been a source of frustration in recent seasons, with line-breaks leading to little and the men in red scrambling to safety - sometimes illegally - but most importantly effectively.
When critics complain about Ireland's lack of an attacking thrust, it's often the battles with Warren Gatland's side that are offered as an example.
There have been long, epic series of phases that have come to nothing and that memory lingers.
Schmidt says the attacking game has moved on and in November there were signs of real progress.
With the weather forecast predicting cold, dry conditions this is a chance to go out and prove that things have moved on.
Concerns exist about the lack of a genuine second playmaker to take the pressure off Sexton and offer variation.
Joey Carbery remains firmly the reserve out-half and there's no suggestion he'll come on elsewhere in the backline in a Beauden Barrett-style impact role, which seems a waste of his talents.
For all of the focus on Ireland's attack, it could be their own defence that makes the difference.
Andy Farrell will back his team to get up off the line and into the Welsh faces, flooding the channels around Dan Biggar who has missed a good chunk of recent rugby and who they picked off several times in last season's game.
How Biggar fits into Wales' evolving attacking shape could be the key to their game.
Much has been made in the press about the Scarletisation of their game, with Ireland keen to attribute the improvements to the much-maligned Rob Howley, but there are clear improvements in the way they go about their business.
Perhaps what will concern Ireland most will be their ability to access possession and territory against a team whose discipline has been excellent.
Wales gave away just two tries at Twickenham a fortnight ago and given Schmidt's side like to strike off set-piece, one wonders whether the opportunities will arise.
They will go to the air, but know that they have gotten very little change from today's opponents who boast plenty of threats of their own.
And when the game breaks up, there is danger out wide where Liam Williams and Steff Evans lurk at a venue where they tore Munster apart in the PRO12 final last May.
At the outset of the tournament, Gatland's decision to state his belief that his side would finish as winners raised eyebrows but his team have backed him up with two strong performances.
They're relying on a favour from Scotland, France or Ireland against England, but that won't count for anything if they can't win in Dublin today.
On Gatland's 100th game in charge, against the team he and they most like to beat, there is plenty of incentive for a performance.
If Wales show up and perform, then the ball will be in Ireland's court.
How much have they progressed? Have they learnt the lessons of the past? Is their depth as strong as they hope? Can they break a world-class defence down?
The 80 minutes today will help address at least some of those issues and teach us plenty about the progress made during this World Cup cycle.
But Japan doesn't matter right now. This Six Nations is there to be won and Ireland come in as unbeaten table-toppers and seven-point favourites.
If they can beat Wales today, they'll inch closer to that Grand Slam shoot-out at Twickenham everyone's been hoping for.
It's the biggest game of the season yet, for six of the starting XV it's the biggest game of their lives.
Ireland's home Six Nations record is on the line, as is their bid for a clean sweep.
Wales just look the stronger side as kick-off approaches, but it promises to be one of those afternoons where every moment seems significant and it goes right down to the wire.
Having endured Paris, Irish fans will be wondering if they can take much more drama but this tournament shows no signs of letting up.