Harrington was a different animal. He first won The Open at Carnoustie 18 years ago, successfully defended the title at Royal Birkdale and then went to Oakland Hills and won the US PGA Championship.

Three major wins in 13 months. "Padraig got the ball rolling," said McIlroy, who made his Open debut in 2007 and finished as the leading amateur. "I think the other Irish players looked at that and that gave them belief."

Harrington, who last month won the US Senior's Open for the second time, continues to possess a competitive edge that sets him apart. "I did two things," he said.

"One, I always talked about majors, that I would win plural majors. I talked myself up.

"But I think the second thing came a bit with my personality. A bit instinctively, I didn't realise I wasn't meant to win.

"Whereas the guys who went before me thought, no, an Irish guy can't do that, I didn't have any ceiling on what was possible.

"That's how I got through the amateur game. That's how I got through at all stages because I wasn't always the most beautiful swinger of the golf club or anything like that.

"So a lot of times I succeeded by purely not knowing any different, keeping my head down and doing my thing and I think that's really helped me."

And Harrington's major wins had a ripple effect on geographically his closest colleagues. Graeme McDowell won the US Open at Pebble Beach in 2010, and McIlroy kept the trophy in Northern Ireland the following year.

"I would have helped the following on Irish guys," Harrington said. "They could say, 'hang on a second, we were number one in the amateur game in Ireland. We played with Paddy. We know what he's like. I can do that.'"

The ripple became a wave as Dungannon's Darren Clarke won the 2011 Open at Royal St George's and McIlroy claimed three more majors, including The Open by the end of 2014.

The 1998 Good Friday Agreement - the deal that brought an end to 30 years of conflict in Northern Ireland, known as the Troubles - opened up further possibilities.

McIlroy told me: "That momentum that we all had was there at that period of time and in conjunction with the R&A looking at Royal Portrush to potentially host the Open Championship again.

"And then for it to go there, I think it's [down to] Irish golf and the players that have come through and how well that we've done.

"But I also think it's a great representation of how far Northern Ireland has come in the last 30 or 40 years.

"Because in the 70s and the 80s and the 90s, no one would have dreamed of hosting an Open Championship in Northern Ireland in those times."

Harrington agreed. "Once it went there, we knew it would be a success," said the 53-year-old Dubliner.

"We'd played a lot of amateur golf up there. The crowds love it and come out. The town gets behind it. So we knew we just had to get it there and everybody would see how great it was."

After the 2012 Irish Open was staged at the Antrim course, attracting huge numbers and player praise, the R&A intensified its feasibility studies.

"There was a bit of work behind the scenes talking it up, emphasising the point of how good it would be," Harrington said.

Within seven years the dream became reality with the first all-ticket Open and nearly a quarter of a million fans flocking to the links.

"You're still a little bit nervous that you want it to be a success," Harrington added. "We knew the crowds would turn out, obviously you have to get the logistics and make sure the crowds have a great time.

"I don't think we could have anticipated how much, but maybe we should have. The players really loved it. Everybody who travelled in loved it."

Never mind that McIlroy missed the cut, County Offaly's Shane Lowry surged to a tumultuous six-shot victory. Tricolours flew triumphantly in loyalist marching season territory, amid unbridled sporting joy.

"We certainly have one of the best Open venues now in Royal Portrush," Harrington reflected. He knows how much The Open means, regardless of which seaside venue holds the championship.

"You could travel 50 miles away from the course, and pull into a petrol station and the person behind the counter is likely to start talking to you about the Open Championship, actually likely to ask you if you have any tickets.

"You can often go to a tournament in the US, and half a mile down the road at a petrol station they don't even know the event is on.

"Portrush is exceptional at taking ownership of the event, believing that it's their Open and the community comes together for their Open and they make it very special."

Harrington says Lowry's win in 2019 was "the icing on the cake" for the competition's return to this spectacular part of the world. And coming back again within six years is "no surprise".

In a delicious twist, the return coincides with McIlroy ending his 11-year major drought by winning the Masters to complete the career Grand Slam. The Northern Irishman is here wearing the coveted Green Jacket.

No further hype needed. "Yeah, poor Rory, everyone seems to build up the pressure on him being the favourite," Harrington said.

"But if you want to be at that level the pressure's always going to be on you.

"Clearly, he knows Portrush very well, he'll have the support and there's no doubt we'd love to see an Irish winner."

But Harrington says McIlroy should maintain some perspective for what could otherwise be an overwhelming week.

"Him going with the Masters' jacket, I think it's enough for him to just swan around and wave to the crowds," said the three-time major winner.

"He doesn't have to win. The people always want him to win the next major or whatever, but it doesn't have to be this one.

"I know it would be nice to be Portrush, but he'll win plenty more majors."

Regardless of whether Portrush can serve up another domestic fairytale, this will remain a golden period for golf on the island of Ireland. How does Harrington think the sport's historians will reflect on it in years to come?

"Clearly it's been unprecedented," he said. "There's been a lot of 'how did we do it?' You know, I don't know if you can replicate things like that.

"Everybody's been trying to find the formula, did we have something special in Ireland? I'm not sure.

"We gained some momentum. We did our thing. I think it's good for us going forward that we will have players who will believe in themselves."

They will do so while speculation grows that new ground will be broken by the R&A taking a future Open to Portmarnock in the the Republic of Ireland.

It is another indicator of how far and how quickly golf in this part of the world has moved. "Definitely, that's a big step," Harrington said.

"It's tried for a long time to lose the tag as the British Open; it's The Open," Harrington said.

"And it represents everybody, not just the people in Britain, but it represents everybody around the world who plays golf.

"It's everybody's Open." But this week with a discernible Irish hue.