Duffy has taken rocky road to the top but stairway to World Cup is at his feet
It took the presence of John Stones beside him in central defence for the reserve team for Shane Duffy to realise his time at Everton was up, but he never gave up on his pursuit of an Ireland spot.
Six years elapsed from the point of the Derryman receiving his first senior call-up to getting game-time in a competitive setting. And 18 months on from that stellar showing against Italy at Euro 2016, Duffy is now the centre-back Martin O'Neill can least afford to be without.
The Brighton and Hove Albion man, all 6ft 4ins of him, will stand tall in the Parken Stadium, well positioned to elevate himself into the coterie of great Ireland centre-halves most recently added to by Richard Dunne and John O'Shea.
Should Age Hareide stick to type by mobilising a direct bombardment as his means of attack tonight - by fielding either Nicklas Bendtner or Andreas Cornelius, possibly both, up front - then Duffy will be in his element.
Such is the defender's reliability to the Ireland cause nowadays that Callum O'Dowda, watching from the bench, wore a broad smile at Wales resorting to long balls in the latter stages of the concluding Group D fixture last month.
Not that Duffy is pigeon-holed an aerial warrior as he maybe once was.
When he spent his final season with Everton on loan at Yeovil Town, his struggles on the pitch were in keeping with the team's wretched sole season in the Championship.
Crude fans, both home and away, were partial to comparing the towering defender to Bambi on Ice when an array of nippy strikers exposed his shortcomings on the deck.
"Back then, after I first came into the job, Shane didn't impress me very much when I watched him," explained O'Neill recently. "He's come a long way to where he is now."
Apart from the brush with death he would overcome on that first senior international gathering in 2010, Duffy's international career has endured a couple of pit-stops. His return to the Ireland fold with the U19s, following surgery and a period of recuperation, was short-lived.
Paul Doolin, newly appointed to the post, axed Duffy from his squad after just one trip - and it couldn't have been for footballing reasons because he'd helped them qualify from the first phase of European qualifiers in Bulgaria.
Perhaps the clue came from Robbie Brady's response to being asked about his first impressions of meeting the newcomer: "A funny man," said the midfielder.
Duffy is known for being a brash character around camp and his personality isn't to every manager's taste.
"I don't think Paul liked me from the very start," wondered the exiled centre-back. His hiatus didn't last long and soon he was captaining Noel King's U21 team, reunited with Brady, who by that stage had also been ditched by Doolin.
Like his club trajectory at that time, Duffy had a mix bag in a green shirt at that level. Scoring an early own-goal against Italy at the Showgrounds during a UEFA qualifier and getting taken to the cleaners by Ciro Immobile might have rattled him but the Northerner recovered to help eke out a 2-2 draw.
Though the senior squad were in Poland at the time preparing for the 2012 Euros, Duffy wasn't much nearer to the shake-up by the time Ireland qualified again three years later.
Admitting this week he resembled an impostor joining in the celebrations, the giant hadn't played a second on the qualification route but had begun to sort out his club status.
He'd been a year at Blackburn Rovers then, having cut his ties with Everton knowing Stones had jumped ahead of him in the eyes of Roberto Martinez. Departing Goodison, where he'd played alongside Eunan O'Kane and Karl Sheppard for the reserves, was a difficult but necessary career change.
Consistency at Blackburn attracted attention from higher-calibre outfits in the Championship, leading Brighton to swoop.
Duffy was part of their promotion from the Championship and survived a summer squad upgrade to retain his status as defensive mainstay.
Throughout his stay at the AMEX Stadium, he has remained a guaranteed starter for O'Neill. Call them the Hills of Donegal, given both their fathers were born in the county, but Duffy and Ciarán Clark have formed a partnership that could endure for another couple of campaigns.
In the meantime, Russia is within sight. Duffy's height allows him to see and sense danger before others and all the ingredients of the package he's become will be demanded over these two legs to get there.