It is estimated that 25,000 Irish fans are tracking 'everyone's favourite second team', as Brian O'Driscoll side have been dubbed in the local media.
They easily outnumber any other travelling contingent, though their ranks have been swelled by swarms of Kiwis who have leapt on board the green bandwagon to enjoy the carnival atmosphere.
Overflowing camper vans covered in the national flag have pursued Ireland throughout the World Cup, but the noise and colour reached a crescendo during Sunday's crucial victory over Italy.
O'Driscoll described the atmosphere at Otago Stadium as better than the Aviva Stadium, while Azzurri coach Nick Mallett said it was like playing at Lansdowne Road.
The support has lifted the team during their march into the quarter-finals and O'Callaghan admits the squad are feeding off their army of followers.
"The fans have been incredible. Maybe that's because the kick-offs are in the evening," said the Lions second row.
"You know the type of crowd we're attracting - they're probably after a few pints. It's an unbelievable atmosphere.
"We're hearing that it's exactly like that at home. There's a buzz about the team.
"There's an awful lot of goodwill towards the team and that puts pressure on you to perform.
"When you're in the team bus and all you pass is green, then when you arrive at the stadium... it's like the Aviva Stadium on crack!
"It's probably the crowd we have, everyone backpacking around and trying to live it up.
"It's a chance to identify with home and show you how proud you are of your country.
"I can't believe how much it's snowballed. Eden Park was incredible, but I couldn't believe Dunedin. It was an amazing atmosphere.
"It lifts you. You feel like Irish people are doing anything they can to get their hands on tickets, and the team is responding to that."
The procession will return to Auckland if Ireland dispatch Wales in Saturday's keenly-anticipated quarter-final at Wellington Regional Stadium.
Victory would sweep them into the last four of the World Cup for the first time and O'Callaghan is eager for the squad to claim a unique place in Irish rugby history.
"This is it, this is the world stage and this is where we want to be," said the 32-year-old Munster forward.
"This brings with it the pressure that you must bring everything for the full 80 minutes.
"Massive games at the World Cup tend to be a bit more stop-start, but when they're on they're full on. One mistake and you're punished.
"We're at that level now and we have to bring our best, hitting around 90-100%.
"To go through is new territory. I'd love it if we stood apart from other Ireland teams.
"I want to look back and see something that separates us from the rest and we need to advance to the next round to achieve that.
"We want to make our time in the shirt different to anyone who has worn it before. We have a good group of players who want to do well."
While the stakes are high with both teams desperate to reach the latter stages of a World Cup that is wide open, there will be no animosity between the rivals on Saturday.
Many of the Irish and Welsh players struck up friendships during the Lions tours of 2005 and 2009 that endure to this day.
"There's good rivalry between us. We know each other so well, as opponents and team-mates with the Lions," said O'Callaghan, who travelled to New Zealand and South Africa with the Lions.
"I really enjoy their company and on a Lions tour if you were lucky enough to be at a Welsh table you were spoilt because it was always good fun.
"Every one of the Welsh lads we hung around with were an incredible crack, incredible fun.
"I roomed with Andy Powell which was an experience, especially when my wife stayed over one night!
"But he was good enough to bring her back a McDonalds, so he was a good team-mate!"
Referring to Ireland winger Tommy Bowe, who joined Welsh region the Ospreys in 2008, O'Callaghan added: "There's bit of a bond between the Welsh and Irish.
"No one else would have accepted an eejit like Tommy Bowe, so fair play to the Welsh lads for taking him off our hands!"
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