Standing tall: Rob Kearney has emerged as one of the stars of this year's Six Nations and is a contender to make the test team for the Lions this summer

He had tickets. Indeed, he was one of the few people in Ireland for whom tickets for the England game in Croke Park weren't a problem. While every Tom, George and Popey in the country was arm-wrestling the last vestiges of hope and favour out of friends of friends of friends, Rob Kearney could put his hands on tickets as readily as he could catch bread out of the toaster. But he didn't use them. Wouldn't. Couldn't.

At the time, they seemed like the worst of days. He was 20 years old, living out the half-life purgatory of weekday squad sessions and matchday nothingness. He was part of Eddie O'Sullivan's squad up until the inevitable Tuesday or Wednesday announcement that he wasn't part of the 22. And he'd leave the hotel feeling hollow that afternoon, knowing that days stretched before him in which to stew. He was more than nothing and yet less than something. Involved but ignored.

"I was getting closer and closer to the team at the time," he says, "and I just found it very difficult around then to go and watch. Just out of pure frustration. I love everything about matchday. Those are by far the best 80 minutes of the week and all the build-up to the game on the day of the game is what I love. It's what everything is geared towards. But I was in the training squad so I was training away with all the lads on Monday, Tuesday and Wednesday and then I had to leave the hotel and go and sit on my hands for the rest of the week. So you're so close to it but then come matchday, you're just another one in 80,000. I just found that very frustrating and hard to deal with so I didn't go."

The upshot is that next Saturday will be, as he puts it, his "first taste of the massive occasion that is England in Croke Park" and the notion of him being just one in 80,000 has long since been consigned to history. If right is right and the world is round, he won't be melting back into civilian life for another decade or so. Chances are, he won't have a whole lot of trouble laying his hands on tickets then either.

You forget that he's only been around a couple of years, that he won't actually turn 23 until next month. The smoothness and confidence that draws your eye to him on the pitch is just as evident off it, so that you're left with the impression of someone far from a typical 22-year-old. As a for instance, the revelation that he'd been putting down his evenings over the past two years finishing off his economics degree in UCD doesn't particularly come as a surprise. He finished it at Christmas, as much to keep the mother happy as anything he says. He thinks that maybe he'll never use it but it's there anyway, a rainy-day fallback should injury someday suddenly decide to pull the rug out from under his life's dream.

You live, you learn. The two are bound together for him. He played his first match for Leinster as a 19-year-old one Friday night against the Ospreys. He stood out on the wing and less than 20 seconds into the game they tossed him the ball for him to clear it into the stands. His first ever touch in professional rugby and a blew it. He went to kick and it was charged down. Last man. Try Ospreys. Welcome to the world.

But you know what? Guess how many times he's been charged down since? Once. In three and a half seasons for province and country, it's only happened a single, solitary time. Live. Learn.

That night was a life-changer. It was as if he'd been looking at that old optical illusion picture where although you're sure you're seeing two faces, it's actually a candlestick that's staring back at you. The game was still the same one he'd been playing since he was a boy and all through his schooldays at Clongowes, inasmuch as it was still called the same thing and the ball was still oval. But everything else took getting used to.

"The physicality was an enormous change for me. When I played my first game for Leinster, I hadn't even played an AIL game. So I went straight from playing under-19, under-20 rugby to playing in the Celtic League. I remember taking a hit off Sonny Parker in the first 10 minutes of that game and he just nailed me into the ground completely. I just remember going, 'Holy God, what am I in for here?' But your body learns to cope with that kind of hit very quickly. It has to, I suppose. I've taken harder hits since."

Including, as we all saw, the one delivered by Italian full-back Andrea Masi after 15 seconds of the game in Rome last Sunday. That the Italians came out swinging with lead in their gloves surprised nobody, the players least of all after they'd spent the whole week telling anyone who'd listen that this would be the toughest game of the championship. They were ready for it and they dealt with it. Kearney got levelled by Masi's clothesline but watch the tape again and you'll see that he was on his feet in a flash and it was only once the whistle went that he crouched down and waited for attention. No histrionics, no drama, no quarter given.

"You don't have to talk about it [beforehand] or say it expressly. It's an instinct. Unless you're hurt, you shouldn't stay down. It should be in every player's psyche. I don't know how I wasn't hurt. I suppose in the heat of battle, the adrenaline is going. I think I was probably lucky – another couple of inches higher and it could have broken the jaw. But that didn't happen and thankfully I was fine. I was a bit dazed for about five, 10 minutes afterwards but nothing lasting. Bit of a sore jaw for the week alright but nothing too serious. I was able to finish the game which was all I cared about."

The Italians didn't do anything by accident though and the next ball that came Kearney's way dropped from the clouds to test him. He caught it, beat a tackler and offloaded. That trick might work on plenty of other full-backs but not Kearney, whose strength under the high ball is more or less his signature move at this stage.

The basis of it he puts down to his GAA background, having played at midfield for Cooley Kickhams and the Louth county minors is his final years at school. At one point, he actually took part in a few of the underage international rules matches against Australia, although in flirtation terms it was more a half-hearted text than chocolates and roses. But that part of his life taught him about timing his jump and keeping his eye on the ball and there's a little of it in every catch he takes now. And don't for a minute imagine he doesn't get a kick out of taking them in Croke Park.

All in all, he couldn't be happier with how life turned out. He can look back now on those days he spent on the periphery of the squad and smile at the depths of the frustration he felt. He can shake his head a little at the effrontery of the 20-year-old him who couldn't take being left out, who couldn't face sitting in the crowd like an ordinary punter. And yet he knows absolutely that without the bullishness of that 20-year-old, without the torture of the time, this 22-year-old wouldn't be who he is or where he is. Leinster full-back. Ireland full-back. In the coin-toss for Lions full-back. He'll take that.

"It has its ups and downs and I've definitely gone through phases where my performance wasn't exactly where I wanted it to be. Those times can be hugely frustrating. You start to question your ability, not your existence exactly but why it is you're doing what you're doing. You start to wonder is rugby actually for you at all.

"But then you have the upsides when your team is winning and you're performing well. You relish the results in life. The things you achieve that were hardest to achieve are the things you relish most. You always say after 80 minutes that the game you win by two, three, four points against a quality, tough side that you've really had to battle for – they're the wins that you enjoy most."

Post one of those next Saturday and talk of the championship will bubble over. Post a runaway one and the Grand Slam momentum will be undeniable. Heady times. A long way from the charge-down against the Ospreys.