O'Callaghan first encountered his future Ireland team-mate when McIlroy was still in primary school and was struck by the notion of meeting a world champion of any age.

"I remember thinking, 'like not just Northern Ireland? Not just Ireland? The best in the world?' And it's just this kid who's grinning from ear to ear," he told BBC Sport.

That was before he saw him with a club in his hand.

"He swung the same way he does now. You're watching him with his little cut down driver or cut down seven wood and he's just never missing the centre of the club face.

"Always right on the button, no toe slappers to the left-side of the fairway. Just flush."

O'Callaghan would become both a friend and regular playing partner in the years that followed and recalls being "flattered" to be paired alongside McIlroy and another pal, Stephen Crowe, at the North of Ireland Championship in July 2005.

On the first day of the prestigious competition, a level-par score around the adjacent Valley course meant O'Callaghan had the better of McIlroy by a stroke. Then came the round that changed it all.

McIlroy had first played the famous Dunluce links to celebrate his 10th birthday. Around the same time he was interviewed by BBC NI and stated his aim to win 'all of the majors'.

His three under par on the front nine of the second day at the North was noteworthy, but hardly foretold a round that would go into lore.

"I was focused on my own game for the front nine and playing quite nicely. I was kind of in it," says O'Callaghan.

McIlroy then eagled the 10th to move to five under.

"Walking down the 11th, they've moved the course around a bit now, but it was a par three down the hill, and I remember him hitting a nice shot in and Stevie turned to me and said 'Rors has got it going today' and, to be honest, I'd hardly noticed.

"I knew he was playing well, but I didn't realise he was five under.

"From about then I stopped thinking about my game and started looking at what he was doing."

Along with virtually everyone else on the course.

A quirk of the draw ensured the course record holder Randal Evans was in the group behind, given a front row seat to see his mark bettered by three strokes.

Like O'Callaghan, Evans had known McIlroy for years by the summer of 2005.

That day in Portrush he remembers being struck by a change in the prodigy.

"He dressed differently than everybody else. He dressed as if he was already a tour pro," said Evans.

"He just looked class. Wearing white shoes, white trousers, the white t-shirt, this pink belt and hat.

"That only comes from being that good and that confident."

As word spread about McIlroy's score, the watching crowd swelled around the group with his playing partners feeling increasingly like spectators themselves.

"Rory would have had a following anywhere in Ireland. Wherever he went at that point, if you got drawn with him, there might be 20 guys there, just people from the club who wanted to watch him play," said O'Callaghan.

"We started with those 20 or 25 guys, but people were obviously calling each other and saying, 'McIlroy's got it going here, you need to see this'. It went from 20 to 50 to 100 very quickly.

"Coming down the old 16th, there was just this wave of people coming towards us. There must've been 1,000 people and Stevie and myself, at this point were just passengers in the round."

When he set his own record, Evans remembers feeling the nerves as he reached the 18th needing a par to eclipse the then standard set by future three-time major winner and Ryder Cup captain Padraig Harrington.

In contrast, as the pressure ramped up, O'Callaghan sensed McIlroy relished the attention.

The 36-year-old made headlines at Augusta this year for not addressing his final-round playing partner Bryson DeChambeau, but O'Callaghan saw the same singular focus even then.

"The more people that were there, you could see his focus go to the next level, he was now in his element," he said.

"It wasn't that he was showing off, but like he was going to entertain and really put on a show.

"He got into that zone or flow that people talk about and you could probably throw an orange at his head and he wouldn't blink."

If Evans felt McIlroy looked like a tour player that day, O'Callaghan remembers him celebrating one like too.

Five consecutive birdies to finish his round gave the watching crowd what they had hoped for with a monster putt on the 18th providing the exclamation point to the piece of history.

"It was one of those kind of moments that we were watching on TV at the time with Tiger Woods where as soon as the putt left the blade, you could just see this thing like a magnet to the hole, roll after roll after roll," he said.

"Rory, you could see it in his eyes. He knows that he's making it right off the bat, and he's got the putter up and he's ready to go with the fist pump, giving it the whole lot.

"It didn't even touch the sides of the hole. Electric."

McIlroy was interviewed the next day by BBC NI and said it was the first bogey-free competitive round of his life. In 2019, he said he could still remember every shot.

From the 17th, Evans heard the cheer that signalled his record had been beaten with the navigation of the course ensuring there was to be an almost figurative as well as literal passing of the torch.

"He had to come walking down past me and I says '61?' and he just says 'yeah, Randy', like why wouldn't he be shooting a 61 round there, you know?" said Evans.

"I shot 64 and that was years of practice and a good round of golf. Think of all the great golfers who have played Portrush and not beat that, tens of thousands of rounds a year.

"Rory was 16 years of age and shot a 61 at his leisure. Cockiness or confidence, whatever it was was just oozing out of him."

Of course, McIlroy would later learn that the game of golf will dent even the previously unshakable confidence of the very best. For all his talent and successes, McIlroy connects with fans in part because of all he has endured alongside his historic feats.

From the Sunday 80 when leading the Masters by four in 2011, through to handing the US Open to DeChambeau last year, with stalled major charges at St Andrews and Los Angeles Country Club in between, there have been plenty of days that could have slowed that familiar stride up the fairways as if there were "springs in his shoes" that O'Callaghan saw even in his earliest days.

Such moments have allowed even one of the most preternaturally talented athletes of his generation to show a human side too.

Perhaps McIlroy was at his most raw after his late run to make the cut came up short when The Open returned to Royal Portrush in 2019. An eight on the opening hole of the championship set an unwanted tone and he was left fighting back tears after his second-round 65 was not enough to hang around for the weekend.

If he beat the course into submission in 2005, Portrush bit back in 2019.

What way will things go as McIlroy returns home this week in search of major number six?

"Rory, obviously, is a different league altogether now," says Evans.

"Missing the cut last time was a massive shock but, Rory's Rory - he'll fix that this time around, I'm sure."