The battle of the half-backs will go almost the whole way towards revealing Ireland's final day destiny.
Lose and they will be limited to preventing England from raising three trophies at The Aviva Stadium.
The prospect of that is a punch to the gut of any Irishman worth his weight in shamrock.
England will strut into Dublin as long as Scotland do not get there before them to swipe the Calcutta Cup at Twickenham tomorrow.
Win and the Irish can still recover the Six Nations championship that was theirs in 2014 and 2015.
Into the bargain, they can do to England what they did to New Zealand in ending their long unbeaten bid for a world-record.
"The human factor in any sporting occasion means that the outcome is unknown," said Ireland coach Joe Schmidt.
"Despite favouritism, despite conditions potentially favouring one side or another or one individual athlete or another.
"That is why people love it because there is that sense of unknown."
The likelihood is the outcome of Wales and Ireland will come down to which half-backs can take advantage when they have field position and which can best relieve pressure when they are under it.
Ireland's Conor Murray and Jonathan Sexton are estimated to be ahead of Wales dangermen Rhys Webb and Dan Biggar in the British & Irish Lions pecking order.
A lot can happen to change all of that over the undulating course of 80 minutes of brutal intensity under the closed roof of The Principality Stadium.
For instance, Sexton's endurance has not been solved just because the out-half came through unscathed against France.
It could even come down to the impact of Paddy Jackson or the will o' the wisp Sam Davies.
The Osprey Biggar was straight forward in his appraisal of the man in the other number ten shirt.
"Johnny is a world-class player and he runs the show for Ireland,' said Biggar.
"Whenever you play against Johnny if you are not 100 per cent on your game, it will be a difficult afternoon.
"That is the biggest compliment I can pay to him. It is up to us as a team to make his life as difficult as possible."
The Welsh wizard is not short of admirers either. Schmidt spoke in glowing terms about his many and varied attributes.
The Ireland coach even used Biggar as an example of how the Welsh aren't as predictable as they once were.
"I don't think you can ever say Dan Biggar is predictable.
"He's got such a big of tricks. He carries very well. He has a fantastic short, high, long kicking game."
Inside Biggar, Rhys Webb has been Wales' best player in this Six Nations alongside Ross Moriarty.
The low centre of gravity and warp-speed acceleration make him a defender's nightmare.
The complete contrast with Murray physically and style-wise make this a more difficult match-up to gauge.
The Munster scrum-half is the total package in that he can do everything well whereas Webb is more of a threat as an individual.
"I am not one to get too caught up in individual battles, my effort and concentration go to the team," said Webb.
"Conor and Johnny Sexton are a big part of the Irish team.
"They have a good kicking game which we have to be aware of and we need to apply lots of pressure to them.
"I focus on myself no matter who I am playing.
"If I do the best I can, I hope the rest takes care of itself."
There are the mini-unit battles all over the pitch, all devised to contribute to the overall goal.
For all the attention spared for the back row argument, they are likely to cancel each other out as Stander, O'Brien and Heaslip battle it out with Warburton, Tipuric and Moriarty and the impact characters that will enter into that warfare.
There will be no quarter asked or given in what is the best rugby arena in the world .
"We play the game to win," said Webb.
"We felt dominant as a team in the three games and have been unfortunate in the last two when results have not gone our way.
"I am taking each game as it comes and trying to enjoy it and play with a smile on my face."
Ireland will look to wipe it away.
Wales: L Halfpenny; G North, J Davies, S Williams, L Williams; D Biggar, R Webb; S Evans, K Owens, T Francis, J Ball, AW Jones (capt), S Warburton J Tipuric, R Moriarty.
Ireland: R Kearney; K Earls, G Ringrose, R Henshaw, S Zebo; J Sexton, C Murray; J McGrath, R Best (capt), T Furlong; D Ryan, D Toner; CJ Stander, S O'Brien, J Heaslip.