It is up for grabs now. Wales host the Republic of Ireland at the Cardiff City Stadiumon Monday with each set of supporters still a little dizzy after several days’ performing needless mental acrobatics in a bid to understand the permutations required for their side either to qualify for Russia as group winners or to make the play-offs as one of the eight top second-placed sides.

Scotland’s draw against Slovenia means those furrowed brows, pub arguments and spreadsheets were in vain. In a group led by Serbia, with Wales in second place and the Republic third, things could scarcely be more simple: victory for either side in Cardiff will guarantee them at least a play-off place, while a draw could still be enough for Wales depending on Croatia’s result in Ukraine. Should Serbia lose against Georgia in Belgrade, the victors in Cardiff will top Group D and qualify automatically for Russia. Defeat is unthinkable.

“It compares favourably to a lot of big matches I’ve been involved in,” said Martin O’Neill, who has just signed a new contract that will keep him in the Ireland job until 2020. “We’ve put ourselves in a position where we have to win the game and Wales probably the same. It doesn’t bother me if Wales are favourites because they have to win. They are at home so they have that advantage. It’s all to play for. The most important is to win the game. Whatever game plan we have we will utilise it the best we can. There is a bit of everything in it. You need patience, but not too much, and after 90 minutes we have to find ourselves in front and we’re capable of doing that.”

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Each side will have to do so without key personnel. Wales overcame Georgia without Gareth Bale but are likely to face a stiffer test against Ireland without their one elite-level player, who will be in Cardiff to cheer his team-mates on but remains sidelined with a calf injury. “He’s a top-class player and will be missed by Wales,” O’Neill said. “He’s world-class. But does it make it easier? Wales have coped without Gareth Bale before.”

The Wales defender Chris Gunter has also been at pains to stress that Wales are not a one-man team. “Whoever is out, those 11 players on the pitch will know what they can do, what they need to do,” he said. “There’s no fluke. Regardless of who’s playing, we’ll have good feeling. Gareth’s been with us, there’s no message. There’s no secret to what we do. There’s no secret, there’s no difference to him.”

Ireland have big-name absences of their own but look certain to welcome Robbie Brady and James McClean back from suspension after their comfortable but decidedly unimpressive win over a poor Moldova side. They will, however, line up without the forward Jonathan Walters and full-back Séamus Coleman, a player O’Neill cites as being as important as Bale and one who continues his rehabilitation from the ghastly leg-break he suffered as a result of an appalling tackle from Neil Taylor in the corresponding fixture in Dublin, which ended scoreless.

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Much as all involved might deny it publicly, that unsavoury incident could lend what already promises to be a tense affair some added needle. Even if Taylor is not selected for this match, it would be naive to think there will not be residual ill-feeling between two sides more accustomed to sporting rivalry of a friendlier kind. “I don’t think it will have a bearing,” the Wales manager, Chris Coleman, said. “It will be similar with two sets of players locking horns, lots of contact, our players know that, theirs do. It was a physical game, unfortunate with Séamus and a horrible situation for Séamus, but he’s coming back and not far off. But for us it won’t be a mention. I can’t tell you about Ireland.”

Wales have yet to fall behind in this campaign but have rarely been too far in front either. They have scored more than once in only one of their past seven games in this group, against Moldova in September. Ireland have problems of their own in front of goal, with Walters injured and Shane Long looking horribly out of form and visibly low on confidence. The Southampton striker remains a willing runner who is adept at pulling defences out of shape but he has failed to score in his past 24 matches for club and country. After missing four clearcut chances against Moldova he looked at his wits’ end.

“Centre-forwards get great confidence from scoring goals,” said O’Neill. “Shane missed a couple of chances and he passed that comment to me on the way in about not being able to buy a goal.” The striker’s place is now in doubt, although he may start alongside Daryl Murphy, who has scored six times for Nottingham Forest this season and bagged two for his country in Dublin on Friday night.

Having received an abject apology from the Georgian FA after what was described as a technical fault led to their anthem being cut short in Tbilisi on Friday evening, Wales have taken the idea on board and announced they will be doing something similar in Cardiff. In an epic act of defiance, the Wales team and their 2,000 fans inside the Boris Paichadze Dinamo Arena continued belting out Hen Wlad Fy Nhadau despite the glitch, and the Welsh FA hopes it can duplicate that atmosphere. Music or no music, what promises to be a full-blooded, winner-takes-all encounter is likely to have an extremely raucous soundtrack.