It's the question that will keep Stephen Kenny up at night between now and next March - how do you stop Kylian Mbappe?
England’s performance against France on Saturday - despite their defeat - might offer a potential answer.
For Manchester City right-back Kyle Walker, it will be scant consolation as he heads home, that he put the shackles on Mbappe more effectively than anyone else in Qatar.
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One of the major talking points ahead of Saturday’s quarter-final clash was how his pace might neutralise the Paris St Germain attacker.
And to a large extent it did, as Mbappe endured a frustrating night on a personal level, even if he was all smiles by the final whistle.
England played with four at the back, with Walker in his familiar right-back role.
As for France’s other opponents so far in Qatar, whether they played a defensive four or a three-man back-line with wing-backs, the outcome was usually the same, with Mbappe dominating a fearsome French attack.
Kenny clearly favours a back-three and he is unlikely to have seen anything at this World Cup to change his mind as we head into the European qualifiers.
While England’s back-four kept Mbappe quiet, Poland played a similar system, with Aston Villa’s Matty Cash on the right, but the Frenchman scored twice.
Australia’s Nathaniel Atkinson, the young Hearts man, was on the right of a four-man defence as France scored four times, including one for Mbappe.
Denmark and Tunisia both played a back-line similar to Kenny’s preferred system.
Against the Danes, with Leeds United’s Rasmus Kristensen at right-wing-back, Mbappe scored twice.
And with qualification to the last-16 already wrapped up, Didier Deschamps only handed Mbappe a 30-minute cameo in a 1-0 defeat to Tunisia.
Nice wing-back Wajdi Kechrida managed to keep the French superstar quiet during that half-hour appearance. But that wasn’t a typical display by the defending champions.
So how can Kenny take the wind out of Mbappe’s sails on March 27 in Dublin?
The performance of Walker suggests that pace is a definite requirement. Mbappe, at full flight, can hit a remarkable 36km-per-hour.
Poland and Villa man Cash spoke after his side’s last-16 defeat to France about the toughest aspect of marking Mbappe.
“When he gets the ball, stops and moves, he's the quickest thing I've ever seen,” said Cash.
“When he stands you up and moves he does it really well. He drops the shoulder, goes short then long. I got into a couple of races with him and I did all right.
“Kyle Walker who can play right-back and he's probably just as quick as Mbappe.”
Does Kenny have a speedster option at wing-back?
Seamus Coleman would hardly have fancied himself in a sprint against the Frenchman at the peak of his powers, nevermind at 34 years of age.
And Matt Doherty has plenty of top attributes in that position, but his pace just doesn’t compare to Mbappe’s.
This is a left-field one, but Stephen Kenny does have one option with both wing-back experience and electrifying speed.
But it would be a real gutsy move to tear up everything he has done so far and put Chiedozie Ogbene in at right-wing-back against France next March.
The Rotherham man played wing-back last year at club level, but was moved into an attacking role this season in the Championship.
On Saturday he scored his seventh goal of the season in the Millers’ 3-1 defeat at home to Bristol City.
Ogbene isn’t keen on the wing-back role - and while he was operating there last season with Rotherham Kenny was at pains to stress his preference for the former Cork City and Limerick man as a forward.
But France and Mbappe provide a challenge like no other side that Kenny’s Ireland have faced.
And while Mbappe can hit a top speed of 36km-per-hour, Ogbene isn’t far behind.
Some reports suggest he has been clocked at 33.6km-per-hour.
He also has the energy levels to do a job on Mbappe for a lung-busting 90-minute shift.
Earlier this year Ogbene broke down his running stats at wing-back, when he said: On average I do about one kilometre of high-speed running in each game.
“I cover about 11k, which is a lot when you do it Saturday-Tuesday-Saturday-Tuesday.
“Physically it is demanding… I need to be explosive in every game.”
Ogbene doesn’t need to worry about the Saturday-Tuesday scenario next March.
Ireland will be in friendly action four days before the France visit, but Kenny has already suggested that fringe players would get a run-out in that one.
So, Ogbene could focus all of his energy on Les Bleus and Mbappe.
With everyone available to Kenny, there might not be room for the Rotherham man in Ireland’s attack anyway.
Kenny doesn’t see Ogbene playing in the same attack as Michael Obafemi, with the Swansea man currently favourite for the role, alongside Callum Robinson or a fit-again Troy Parrott.
So, for one night only, and with Mbappe presenting a unique challenge to Kenny’s defence, is Ogbene the answer to the question that is giving his international manager sleepless nights?
It would be a huge gamble. But with the potentially back-to-back World Cup winners coming to town, how much does Kenny have to lose?
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