He still remembers that scene, that match, that day. He was a 23-year-old making his international debut as a 63rd-minute substitute, and it meant everything to him at the time. It still does now. "It was the best day of my life, " he admits without deliberation, quite a statement when you consider what he's been through with Munster down the years.

"I went berserk at the final whistle because I'd done it, I'd played for Ireland. People think that we take things like that in our stride, but not when it's something like your first cap. It was something I'd aimed for all my rugby life and to achieve it was absolutely the best feeling."

You suggest he must have thought back then he'd arrived, he was the man, and the caps would make their natural progression towards his mantlepiece. He considers this one. "I don't think I thought that straight away. It was only probably over the next few years that not starting got to me a little.

There were a lot of tough calls along the way and, without trying to be arrogant, it's frustrating when you know you're good enough to play but somebody's there in front of you."

While he's always seemed content to play the role of Robin to Paul O'Connell's Batman with Munster, dropping down to the part of Alfred the butler for Ireland hasn't pleased him. Malcolm O'Kelly has been the human roadblock that's stymied his progress and the tough breaks O'Callaghan talks about have been varied and many. Like the time the Leinster second row's poor time-keeping during a camp in Spain bothered Eddie O'Sullivan enough to drop O'Kelly for Ireland's Six Nations game against Italy at Lansdowne Road. O'Callaghan took his place in the starting line-up for the game, only for O'Connell to pull out injured a couple of days before the game. Thus O'Kelly and O'Callaghan started together, the more experienced of the duo had the game of his life and when O'Connell returned to fitness, his provincial colleague was back on the bench, where he was to be marooned for another couple of years.

"There's always been so many quality players around me in my position, " he says, safe in the knowledge he's now a first-choice for Ireland, no matter what state O'Kelly's knee might be in. "I respect Mal and any time I've got ahead of him into the starting line-up, I know I must be doing something right. I'd like to think he feels the same. Having no elbow room pushes you on, it ensures you don't sit back and become happy with your lot, it forces you to push on and improve yourself."

Even though competition has held back his international career - as it once did at provincial level when he was stuck firmly behind Mick Galwey, John Langford and Mick O'Driscoll in the pecking order - he wouldn't have it any other way.

"I'm better with people breathing down my neck. I prefer it. Well, that's not quite true, it's just I've never experienced any other kind of situation, I've never really had a chance to walk into a position with nobody around for company. But I like the competitiveness, I think it pushes you on. I've seen other guys struggle with their game because there's nobody challenging for their position within a squad. I have no idea what that feels like - and I don't want to know either."

The work ethic that's sprouted from the competitiveness around him is what defines him on the pitch. If television rugby coverage were to copy the football model and adopt a player cam, O'Callaghan would make for interesting viewing. A 25-yard tackle effort against New Zealand in Hamilton during the summer is the pinnacle of his gut-busting efforts to date, but it's only representative of what he does on a weekly basis. The guy positively thrives on working himself to the bone, which is why the slight dip in Ireland's lineout performances will bother him until things are fixed.

"I remember chatting to Nathan Sharpe after we played them in November, " he says, "and him telling me they changed their lineouts completely because they were playing us.

For us, it's about working hard on altering a few things. Sometimes people expect us to deliver 100 per cent ball straight off the top because we've done it in the past. It's not an excuse, but the high standards we've set have opened us to criticism when things don't go perfectly. Take the Heineken Cup final. After the game myself and Paulie were talking about our line-out that day and how we were disappointed with the quality of the ball we were getting to Strings. That's what it's like working with these guys, you always want to make yourself better. If you go to a Munster or Ireland session sometimes you'd actually wonder if we're friends at all. You can get a bollocking off anybody if you drop a ball or mess something up."

The self-flagellating has stopped a little, now they're European champions and all that, and that success has allowed O'Callaghan to be a bit more positive. "It's given me more confidence in myself, and I think it's helped push my game on a bit, as it has with other guys in the squad. Sometimes I think we can be overly hard on ourselves in Munster, and the same applies to Ireland. Even if we win a game we're always looking at what we can improve upon and what we can change. But sometimes you just have to forget about all that and take the merit you deserve."

He's taken that merit onboard but it hasn't changed his life. He knows Brian O'Driscoll goes shopping at night with his hoody up to avoid being hassled, and that Ronan O'Gara gets a fair bit of attention too, but O'Callaghan himself doesn't get recognised that often.

"Not down in Cork, maybe on a trip to Limerick or something like that, " he laughs. "It's only the hurlers they want down in Cork. I know lads like Rog and Drico are in the public eye and get recognised a bit more but the only people who recognise me are the real rugby people, and that's usually because they recognised my ass rather than anything else.

But the publicity thing isn't really me."

That might surprise a few out there who know O'Callaghan as a prankster but his jokes have always been more about boredom relief than attention seeking. He takes his job seriously and knows exactly what Ireland need to do this afternoon. "We're focused on not making unforced errors, which is kind of unusual for the first match in a campaign, " he says. "Usually it can take a game or two to be able to play without mistakes but we know Wales can hurt us if we gift them easy possession. They're capable of scoring from anywhere and we just can't cough up easy ball."

Follow that policy and he just might be doing another Millennium Stadium dance. He might even get some of his teammates to join him this time.