Brian O'Driscoll is in a droll mood. The serious business of the Lions tour of Australia will soon begin, alongside a more evocative sense that the northern hemisphere's finest rugby player is approaching the end by returning to the country where his extraordinary career really began. The poignant certainty that nothing lasts forever is intensified when it assumes the shape of an intelligent sportsman readying himself for one last defiant tilt at greatness.

Yet O'Driscoll underplays the gravitas of these final and often wounded days. He jokes that, just before one more night-time session on a physiotherapist's table down the road in Dublin, prior to him joining the Lions for his fourth tour in the famous old red shirt, "I'm kept going. They're holding me together just fine."

He laughs softly, seemingly sure that the back spasm which nearly ruined the end of his domestic season will not affect him in Australia as he strives to secure the series victory that has eluded him with the Lions. O'Driscoll points out that we have an hour before his back gets kneaded and tended to again, and so there is plenty of time to enjoy his reflections and anticipation. The fact that he has just decided to play on for another season has also diluted the scrutiny that could have engulfed him.

And so, before a warmup game against the Barbarians in Hong Kong on Saturday, O'Driscoll amuses me with a story of how he found out that he had been selected for this tour. Before he led the Lions in New Zealand in 2005, when he was badly injured by a spear tackle in the opening minutes of the first Test, O'Driscoll was told privately of his appointment. This time, like everyone besides Sam Warburton, the new captain, he waited for the official announcement on television.

"The only problem was that, with so much going on at home, I had my Sky Plus paused by a minute. I'd just reached the moment when the full-backs' names were read out and … [O'Driscoll pauses dramatically] I got a text from my sister. It said: 'Get in!' All I could see on the telly was Stuart Hogg [the Scottish full-back] and I thought, 'Oh … all right.'"

O'Driscoll sighs wryly. "So … Sky Plus isn't always a plus."

Yet it does not take long for O'Driscoll to articulate the private meaning of this tour for him. "It's probably the last big thing I want to be involved in," he says. "I'm definitely not going to be involved in the next World Cup. So this is my last big moment in rugby."

Paul O'Connell, another great Irishman and former Lions captain on his final tour, was equally thoughtful when we met last month. The lock admitted with a wince that he had been forced to concede that past players like Lawrence Dallaglio, Keith Wood, Jeremy Guscott and Matt Dawson could present themselves as "custodians of the Lions". Neither O'Driscoll nor O'Connell can challenge their status for, unlike the 1997 Lions, they have yet to taste a series victory in the red jersey.

"It's true," O'Driscoll says simply. "Matt Dawson made contact, congratulating me, and I said 'Yeah, it's probably about time I won one of these – I've certainly had enough cracks at it.' Until you win a series it's difficult to place yourself in that elite group of great Lions players. It's not enough to produce one-off performances or be nearly-men.

"You've got to win a series to be properly remembered. I've talked to Matt about that dummy over-the-head pass that secured the [first Test] win in South Africa. How many times have people spoken to Scott Gibbs about his big hit on Os Du Randt? These moments are timeless – but they're only timeless because of the victory that followed. To be considered a great and a custodian of Lions rugby you have to achieve that success. So here comes another opportunity to join that elite group."

The idea that O'Driscoll should be considered a lesser Lion than even a fine player like Dawson seems an affront to good sense. But a winning series for the Lions happens far less often than noble defeat. O'Driscoll knows that his iconic sheen would be pegged down for generations if he could rise up one last time and inspire victory over the Wallabies.

He has already given us one of those "moments" which he cherishes. It might only have led to a first Test win against Australia, in a narrow series loss, but the brilliance of O'Driscoll's try in 2001 refuses to fade.

Starting in his own half he slashed open the Australia defence, as if using a magisterial cleaver which left Nathan Grey and George Smith for dead, before an outrageous step took him clear of Matt Burke.

"There's ego in all of us rugby players," O'Driscoll says, "and I've looked at the odd montage of myself and seen that try. I have a wry smile to myself. It was one of the good ones because Australia were renowned for their impenetrable defence. I just got out of Matt Burke's way before he killed me. But I think more about the fact that we lost that series. It was even closer than South Africa in 2009. We should have won the second Test and the series. We carved them open but didn't finish the job."

Australia features large in O'Driscoll's story, for he made his Test debut in Brisbane in 1999. It was the first of his 131 Test appearances – 125 for Ireland and six for the Lions which mean that only George Gregan, the former Wallaby, has played more international rugby.

Ireland picked O'Driscoll even before Leinster did – and it's fitting that Warren Gatland, now the Lions coach, should have been bold enough to play him on the very ground where the first Test of this series will be held.

"Gats has never been shy of picking young guys," O'Driscoll says of the New Zealander who coached Ireland before Wales. "Thankfully, now that I'm at the other end, if a guy is good enough he's never too old for Gatland either. Before that Test debut my sum of high-grade rugby amounted to nothing. I'd only played for the Under-21s and a bit for my club in the second division. So to suddenly face a Test against the world champions and some of my childhood heroes was an experience.

"I remember being frightened when the fireworks went off and they came running out on to the pitch. I looked at their backline and I was startled at how enormous Ben Tune seemed. And then came Joe Roff, Daniel Herbert, Tim Horan, Stephen Larkham … I just thought 'Do I belong here?' Timmy Horan was a childhood hero. He was a great distributor, elusive, good stepper, very physical, defensively very sound. What a rounded player."

O'Driscoll could be describing himself, only forgetting the courage and intelligence that brackets him with Horan, but he is more intent on recalling Ireland's 46-10 defeat. "I could've been a lot better. I remember dropping one ball when I was put into a hole. That's one of my few clear memories and it's a negative one. I did better in the second Test but it was a time when we didn't have systems in place – we defended in ones and twos without any real structure."

Gatland will be much more systematic in his approach to this tour. "He was on a massive learning curve back then," O'Driscoll says. "Like me as a player, he's a drastically different and better coach now."

O'Driscoll's first two Lions tours, under Graham Henry and Clive Woodward, were blighted by managerial mistakes. He is unequivocal when asked if the 2009 Lions, coached by Ian McGeechan, were the happiest of his three squads.

"Without a shadow of a doubt. It was the most enjoyable for sure. But I love Australia as a country and enjoy touring there most. In 2001 we probably trained too hard and that caught up with us in the end. But we had such a harmonious and tight bond in 2009."

This has been a less harmonious season – with Ireland winning only one match in the Six Nations and O'Driscoll sent to the sin-bin, and suspended for three weeks, after stamping on Simon Favaro during an abject loss to Italy. "Maybe there was some frustration in me in wanting to get the player out in that manner. I didn't rake backwards. I came down on him which is obviously illegal. Sometimes you have to hold your hand up and apologise. But the whole tournament is not one I look back on fondly. Losing four out of five [Ireland actually drew against France but were defeated by England, Scotland and Italy] and the one game you win is against the eventual champions [Wales]? It was a bizarre tournament."

O'Driscoll, however, will never forget Ireland's match against England, on 10 February, the day he became a father. "When I got a call at 8.10 on a Test match morning, and I saw [his wife] Amy's number come up, I knew something was going on. It was a shock to the system.

"But Amy was as calm as you like and everything ran like clockwork and I was back in the team hotel by half-twelve having had a little girl. I didn't have a howler but I didn't have one of my better games. I caught myself halfway through the second half and realised there was a Test match going on."

The birth of Sadie is the most obvious example of how O'Driscoll's life, at the age of 34, is changing. He sounds suitably blissful before, returning to the Lions, he says, "little girls have you up at half-six so I've seen a bit of Super Rugby. The Brumbies and Reds have played some good stuff. They'll bring good form to the Tests."

O'Driscoll returned to competitive rugby last weekend, playing the entire 80 minutes of a bruising yet winning RaboDirect final victory against Ulster. His injury, suffered against Glasgow this month, was a familiar back spasm.

"It's happened a couple of times in training when I hyper-extend my back. Some facet joints send all the muscles in my lower back and lumbar-spine into spasm. I couldn't take my boots off after that game – but I'm fine now. I'm also excited – and looking forward most to training with the Lions. It's like a mini-game every day because the skill levels are so high. I love playing with guys who have the ability to think a couple of phases ahead and see opportunities that others at a lesser level aren't capable of doing."

He yearns to be part of a winning series and O'Driscoll's hopes are clear when he considers the merits of his fourth Lions squad. "We've got great balance and the competition in the back row, centre, halfback and front row proves our quality. The Australians came out with these quotes about 'slabs of meat'. But they're going to be unpleasantly shocked by the calibre and range of our skills."

Does the great old warrior believe that, at last, he will be part of a victorious tour? "Absolutely. I absolutely do," he says in a resounding echo, sounding more than ever like a true custodian of the Lions.