Growing up in Dunmore East in Waterford, he juggled athletics with rugby as a teenager, but competing at Munster level with Ferrybank AC, Barr would often be demoted to C or D squads for relays. “He was okay but nothing spectacular,” says his mother, Martina. “I didn’t see anything standing out until he was in about sixth year.”

Soon after starting college at UL, Barr figured he’d had enough of athletics but his dad, Tommy, urged him to stick with it. At an event in Cork, Tommy crossed paths with Drew Harrison who, together with his wife Hayley, would soon take over Barr’s coaching duties at UL.

“I told him he’s one to watch, that he had something special,” said Tommy. “He had all the speed, but his technique was like he was running through sand and I said, ‘once he develops, he’s going to fly.’”

Barr was 18 at the time and had a pedestrian personal best of 56 seconds for 400m hurdles, but once the Harrisons refined his technique he was transformed, clocking 50.06 to win the Irish senior title and finishing sixth in the European U-20 final. That was in 2011, and from there until the Rio 2016 Olympics his progress was linear. All the same, he went to those first Games a relative unknown outside of Irish athletics.

But that didn’t last.

In the build-up, Barr had been a stressed, panicked athlete, a labral tear in his hip sidelining him from training for 11 weeks through the spring. At the Europeans in Amsterdam, five weeks before the Olympics, he crashed out in the semi-final, unable to break 50 seconds. He put a brave face on for the media but after walking through the mixed zone, Barr gave his coaches the blunt appraisal: “That was shit.”

Before leaving for the Olympic holding camp in Uberlandia, he spoke at Dublin airport like a man resigned to his fate, content just to be getting on the plane. But something clicked in those final few weeks, his coaches stripping any fluff from his programme, given the urgency, and focusing on vital specifics: speed and technical proficiency.

Barr shocked himself to finish second in his heat in 48.93 and in the semi-final, he powered to victory in 48.39, becoming the first Irish athlete since Bob Tisdall in 1932 to reach an Olympic sprint final.

In it he produced the race of his life, obliterating the Irish record to finish fourth in 47.97, one 20th of a second outside the medals. The run left him elated, but you can’t come that close to the podium and not wonder how it might feel.

After returning home, he realised quickly life would be different, his face suddenly known by the nation. Barr basked in the deluge of goodwill that flowed his way but the strength of that current also had its dangers.

“It completely changed how I acted and behaved, particularly in public,” he says. “I knew that whatever I was doing, unless I was in a very close setting with friends, in the age of smartphones someone could record something, I could say something and it could be taken out of context. It could be very quickly turned into something negative.”

He’d go to nightclubs with friends and spend half the night obliging selfie requests. He’d hear his name called in the street and expect a familiar face (it almost always wasn’t). There was one small incident that could stand for the whole: Barr was shopping in Lidl and, being the adventure junkie he is, he took off down the aisle hanging off the back of a trolley.

Moments later an old man walked over, shook his hand, and told him to be careful, that he wouldn’t want to hurt himself doing stuff like that. A harmless interaction, sure, but a reminder that the price of fame is that someone is always watching.

“I’m very cautious of how I act on social media,” he says. “It’s being watched a lot more closely than before.” On days when he’s tired, cranky, unsociable, he’ll still put on a face, making conversation with fans, aware of how privileged a position he occupies.

“It did sometimes get draining, but it’s brilliant and that’s what sport is about. You’re doing it for your country, for the people of Ireland who love jumping on that bandwagon when things are going well.”

For most Olympians, sporting careers play out in a chiaroscuro blend of shadow and light, their performances often existing in a publicity vacuum in non-Olympic years. It can be dizzying when the quadrennial spotlight shines their way, but Barr is most at home beneath its glimmer.

“Bring on all the bandwagoners,” he laughs. “The more the merrier.”

There’s a thought he finds hard to escape, not so much because he obsesses over it himself as the regularity with which others put it in his mind. It’s the idea that his fourth-place finish in Rio might eventually be upgraded, with anti-doping authorities able to retest old urine samples for 10 years as better detection methods become available. It has proven astonishingly effective at weeding out cheats long after they made off into the night with their stolen medals.

So far there has been a total of 127 disqualifications from the London 2012 Olympics, with 29 medal upgrades, 14 of them in athletics.

“It’s a real possibility but I’m not getting my hopes up,” says Barr. “Not letting it bring me up or down.”

Since 2017, the Athletics Integrity Unit has launched a major crackdown in areas where drug-testing was historically poor. While the 400m hurdles hasn’t experienced the major doping busts that have occurred elsewhere, Barr isn’t naive enough to think it’s always been a level playing field.

“I’d never think seriously into it because I don’t want to put myself out of the race before I’m in it and without evidence there’s nothing to say anyone is or isn’t doping,” he says. “It’s innocent until proven guilty, but there were times I was stepping on the track thinking, ‘this guy could well be doping.’

“Doping is a step ahead because anti-doping can only be reactive — make tests and protocols for methods already out here — but it’s a matter of catching up on that. Two or three years ago a load were retested from London (2012) and there were a load of busts so there’s no reason that won’t happen in a year or two. While it’s terrible for the sport and the headlines to see negative stuff, it’s good in that the people cheating are getting caught. The anti-doping is doing its job.”

When it comes to Tokyo, Barr knows it will take something far quicker than the 47.97 he ran in Rio to reach the medal stand. His event has moved on rapidly in the years since, with the arrival of new-age footwear and a duo of freakish talents taking times into the stratosphere.

Norway’s Karsten Warholm, 25, broke the world record in Oslo earlier this month, clocking 46.70, while 23-year-old Rai Benjamin clocked a blazing 46.83 at the US Olympic Trials last month. Barring injury or mishap, they should have a lock on the gold and silver medals in Tokyo, though winning bronze would also need Barr to ascend to a level he’s yet to reach.

“The event has exploded,” he says. “I think it’ll take a personal best to get to the final and it’ll take low 47s to get anywhere near the medals.”

Such an achievement may be highly unlikely, but to discount it entirely would be to ignore Barr’s habit of finding an extra gear when it matters most. In 2016 he hadn’t broken 50 seconds before the Olympics yet he ran under 48 seconds in the final. In 2018 he went to Berlin with a season’s best of 48.99 but clocked 48.31 to win bronze in the European final. This year he goes in with a season’s best of 48.39, his fastest ever time outside a championship.

There’s something about the pressure of these events — the crowds, the noise, the almost nauseating sense of occasion — that helps him flourish where so many falter. In 2016 Barr walked out for the Olympic final and waved to Irish fans in the stands with the giddy smile of a guy who’d just recognised his friends across the bar.

That’s how he is when his preparation has gone well. If he’s quiet and subdued in those moments, it usually means it hasn’t. Uncertainty about his fitness breeds nerves. Confidence about it breeds calm. It’s the same when he’s in the call room, the confined area where athletes must sit and stew at length before being released out on the track.

“It can be a very nerve-racking place,” says Barr. “You and seven competitors in a room no bigger than a small hotel room with chairs facing each other and everyone talking to themselves, eyes closed.

“When I’m in shape and got through the heats and am in a final, that’s when I’m in territory where I know what to do; I’m champing at the bit,” adds Barr.

The routine that will carry him around the track and over those 10 barriers is one he’s rehearsed for more than a decade.

“Get out hard out of the blocks, attack the first hurdle,” he says. “That sets up the speed for the rest of the race. Down the back straight I’m on autopilot, I could nearly do those five hurdles with my eyes closed, and then it’s about maintaining fast rhythm, staying relaxed, and attacking each hurdle.”

Barr typically runs with 13 strides between the first seven hurdles before switching to 14 as the fatigue sets in, and to ensure that doesn’t affect his speed he’ll focus on shorter, quicker strides on the final bend. He’s one of the fastest finishers in his event, but the key is staying in contention until they turn for home.

“With 100 metres to go, it’s about keeping these hurdles clear and driving off them,” he says. “Then I completely empty the tank with whatever I have left.”

Recently he set off for the Irish holding camp in Fukuroi and given where he has come from — that average, unexceptional kid who’d have his arms by his side — to be there, preparing to mix it with the world’s best, seems a victory in itself.

He’s 28 now, and the harsh reality is he probably won’t come home from Tokyo with a medal. But to fixate on such rare and precious objects in a truly global sport is to harbour a warped view of what truly constitutes success, knowing where he’s come from, knowing what he’s already achieved.

Maybe Barr will get knocked out in the heats. Maybe he’ll reach the final and do for the second time what no other Irish athlete has done since 1932. Maybe he can even go one step beyond. The only certainty he has is that which he controls.

“All I can do is run as hard as I can,” he says. “Come off the track with my head held high, saying I gave it my all.”