It's not a Grand Slam decider, it's not a series-clincher on the road, it's not even do-or-die, but this one, which will be played out in front of 80,000 souls at the Stade de France, feels big.
"It's always nice to win," said Andy Farrell to reporters on Thursday when he named a side with just one change, scrum-half Jamison Gibson-Park in for Conor Murray, to face the world champions.
Be sure he'll be singing a different song when the door closes on the Stade de France dressing room this evening ahead of the meeting between the world's two highest ranking sides.
Ireland, on a run of 27 wins in 29 games, are the top dogs.
The tangibles of winning: four (maybe five) points and distinct advantage in the race for top spot in Pool B: a record-extending victory, 16 it would be. Ireland could, in fact, qualify for the knock-outs with a bonus-point win, while denying the Springboks one.
The intangible of losing: the bubble burst, the climb back up the mountain steeper for Ireland than for the reigning world champions.
A loss would, Scotland in two weeks' time notwithstanding, put Ireland on a quarter-final collision course with hosts France in Paris. With or without Antoine Dupont, a frightening prospect.
South Africa are one of those teams that are more than the sum of their parts once they pull on the famous green shirt (white tonight).
That said, six of their starting XV ply their trade in Japan. Super Rugby, Champions Cup, URC, it is not. Indeed, another five of the 23 started in the Stormers side beaten by Munster in that final.
It's less than a year ago that Ireland beat South Africa, 19-16, in the Aviva Stadium. Tries from Josh van der Flier and Mack Hansen (above) did the trick in what was the 27th meeting of the teams, who have never faced off in a World Cup before.
Both sides return with 11 each of the starters from that match-up.
Rónan Kelleher, Bundee Aki, winning his 50th cap, Gibson-Park and James Lowe the new men in, while Bongi Mbonambi, Franco Mostert, Faf de Klerk and Manie Libbok are the fresh blood for the Boks.
Indeed, a win for the Six Nations champions would make it three-in-a-row over the Springboks, a triple last achieved in 2004, '06 and '09.
The two sides boast many impressive stats from the first two rounds but, given that Romania were half the opposition, they are practically meaningless.
It's more interesting to note that games between Ireland and South Africa, who opened with an 18-3 win over Scotland two weeks ago, are generally low scoring.
In fact, only one game, the second Test in Johannesburg in 2016, won 32-26 by the hosts, in the last 117 years has produced more than 50 points.
Six points or less have separated the teams in seven of their last 10 meetings.
Much has been made of South Africa's 'bomb squad', the seven deadly Boks, including two Munster men in RG Snyman and Jean Kleyn, who are set to enter the fray in the second half to cause (more) havoc.
Their starting pack weighs 896kgs in total against Ireland's 894kgs, but off the bench they offer 754kgs of bulk to their opponents' 558kgs spread over five men.
"They obviously present a big physical challenge – I think we present a big physical challenge as well – but also a really technical challenge," said forwards coach Paul O'Connell.
"They are a very smart side. That is probably one of the things you admire the most - the smarts they can bring along with their physicality
"It's a strength of our side as well. We speak about this Irish team and what they stand for. They have got to bring everything they have brought to big occasions over the last few years."
It's a calculated risk that Rassie Erasmus and Jacques Nienaber are willing to take to bully Ireland off the field. Even if it backfires, there's a backstop, in the form of the five-match points they'll pick off Tonga in round four.
They have an incredibly versatile backline, also, so it would really need a combination of injuries, cards or HIAs to disrupt their game plan there. For all they do have, a reliable goal-kicker is not among them. Here's where Handre Pollard may be missed.
These things happen, however, and the battle of nerves, patience and skill that is the kicking game, will also have a huge part to play.
Lowe and Hansen, and the ever-reliable Hugo Keenan, have been well versed in that game in recent matches and can match and better Cheslin Kolbe, Kurt-Lee Arendse and Damian Willemse in the war of the skies, while the breakdown battle will be fast and furious.
Ireland, still boasting the fastest ruck speed in the game, will aim to break even at scrum and lineout, so often a gateway for South African inroads.
The Stade de France has been a happy hunting ground for the southern hemisphere side. It was here they won the Webb Ellis against England in 2007, and eight years previous Jannie de Beer dropped five goals to knock the Red Rose out in the quarter-finals.
They have won all four World Cup games they have played in Saint Denis. On the other side, Ireland have won just three, lost ten and drawn once, always against France, at the venue.
On one level both teams can afford to lose this game but on another, the feeling is that a heavy loss would hit Farrell's men harder.
On the eve of Ireland's crucial pool game against South Africa, @OFlynnPaul samples the view in Paris, while @bernardjackman gives his verdict to Clare McNamara #rterugby #rwc2023 pic.twitter.com/3sVUSAlrye
— RTÉ Rugby (@RTErugby) September 22, 2023
Ireland, slight underdogs today, have risen to almost every challenge laid down over the last four years under Farrell, whose 82.5% win rate from 40 Tests in charge make him the nation's most successful permanent head coach.
"We have always found a way and figured it out and it is a real strength of theirs," said O'Connell.
"They are going to have play super well but also figure things out, which is something I really enjoy watching them do when they have a challenge in front of them."
While the rest of the rugby world gets carried along in the fast flowing stream of hype surrounding this fixture, this team might just get on with their job.
They are a confident side, their form and quality can tip the balance.
Verdict: Ireland by four
Ireland: Hugo Keenan; Mack Hansen, Garry Ringrose, Bundee Aki, James Lowe; Johnny Sexton, Jamison Gibson-Park; Andrew Porter, Rónan Kelleher, Tadhg Furlong; Tadhg Beirne, James Ryan; Peter O'Mahony, Josh van der Flier, Caelan Doris.
Replacements: Dan Sheehan, Dave Kilcoyne, Finlay Bealham, Iain Henderson, Ryan Baird, Conor Murray, Jack Crowley, Robbie Henshaw.
South Africa: Damian Willemse; Kurt-Lee Arendse, Jesse Kriel, Damian de Allende, Cheslin Kolbe; Manie Libbok, Faf de Klerk; Steven Kitshoff, Bongi Mbonambi, Frans Malherbe; Eben Etzebeth, Franco Mostert; Siya Kolisi (capt), Pieter-Steph du Toit, Jasper Wiese.
Replacements: Deon Fourie, Ox Nche, Trevor Nyakane, Jean Kleyn, RG Snyman, Marco van Staden, Kwagga Smith, Cobus Reinach.
Referee: Ben O'Keeffe (New Zealand)
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