Ronan and Johnny. They’ve been connected in the public imagination for so long now that it comes as a genuine disappointment to discover that they’re not living together in Paris, rubbing along awkwardly in a poky apartment near the centre of the city, Ronan playing the wisecracking, come-day, go-day Matthau figure to Johnny’s fastidious, buttoned-down Lemmon.

No; crushingly, it turns out that Ronan, Jessica and their children have their own place, while Johnny and Laura live here, in an enormous house in Châtenay-Malabry, a quiet suburb 10 km southwest of the city centre.

“We only have an apartment in here,” he says, as he slips the key in the door. He laughs because he knows what you were probably thinking. No wonder he walked away from Leinster. “Please don’t say we live in a mansion!”

He is good and thoughtful company tonight. A little tired, but content. Or as content as Johnny – it’s actually “Johnno”, but more of that later – allows himself to be in his working life.

Rugby players in France put in a full working week. Even on match days with an evening kick-off, they’re expected to clock in at 10am.

When he was at Leinster, he spent the day of a home game sitting in his apartment in Goatstown in Dublin. Four hours before kick-off, he took out the ledger in which he wrote down his game plan for the match, stared at it for a couple of hours, then set off for the RDS.

But then, life for Johnny and the childhood sweetheart he married 11 weeks ago has been a series of adjustments. There’s the rugby (going okay), the language (coming on) and remembering to drive anti- clockwise around roundabouts (getting there). And, of course the mundane practicalities of turning their modest-sized apartment into a home: involving visits to Ikea and debates on flatpack furniture assemblage.

You quickly realise that Johnny and Laura are just like thousands of other emigrant Irish couples, waiting for the sharp edges of their new world to soften into something like permanence. “The first couple of weeks were really tough,” he says, “especially for Laura. I was at the club all day. I was gone from nine until six. But at least I’ve got somewhere to go every day and something to focus on. Laura was locked up here in this place by herself, and there was nothing here.”

It was tougher still because moving to another country never figured anywhere in their life plan.

At the start of the year, they were two homebirds with a neat, grid-like order to their lives. Johnny played rugby for Leinster, the team he supported as a boy, while Laura was a teacher in Loreto on the Green, the school she attended when they met as 13-year-olds in Rathgar Tennis Club. They liked the easy familiarity of their world, they didn’t want it to end.

Johnny makes that much clear in Becoming a Lion, a fascinating and achingly honest book that follows the sine wave of the last 12 months of his life.

With his contract up for renewal, he and his agent, Fintan Drury, entered into negotiations with the Irish Rugby Football Union (IRFU) and quickly discovered that his employer’s estimation of his worth fell far short of his own and, more significantly, that of Racing Metro president Jacky Lorenzetti, who wanted to make him one of the highest-paid players in the world.

He says he never expected the union to match Racing’s offer, but having played a lead role in three Heineken Cup triumphs, he thought he deserved to be on the same money as Irish rugby’s top earners.

“I always thought I’d be a one-club man,” he says, “but when the first offer [from the IRFU] came in – I mention it in the book – it really pissed me off.

“Look, they’re trying to do what’s best for Irish rugby. It’s a business, but I kind of felt, in the last three years Leinster had won three European Cups and a Magners League and I was wearing the number 10 shirt. I was a big part of it.

“Maybe the IRFU just didn’t rate me. Maybe I’ve too high an opinion of myself – that went through my head, but I always thought, while the negotiations were going on, that it was going to get sorted out and that it was all just a game.

“I remember speaking to Joe [Schmidt, the former Leinster, now Ireland, coach] the night before and he said to me, ‘Can you give me 24 hours?’ and I thought, ‘It’s going to get done.’

“Then I spoke to the IRFU the following morning, before Joe even got back to me, and the way the conversation went was, well, we’ve given you the best offer we can give.”

And that was it. Thanks for everything.

When the end was confirmed, he describes in the book how he lay down on his bed and cried. “It was a weight off my shoulders that it was finally done,” he says, “but I also felt guilty that I was the one to break up the group.”

For better or worse, his life is here now. Racing is a club with big ideas and he is central to them. Their results this season have been mixed – they won four and lost two of their opening six games.

“We didn’t really click as a team until last week,” he says. “We have 14 new players. With Leinster, we signed maybe one or two every year. So it’ll take time. Everyone has been walking around on eggshells a little bit.”

Then there’s the language. The coaching is all done through French. He studied it for his Leaving Cert – he thinks he got a B3 or a C1 – but that was a few years ago now. He and Laura try to do two hours of lessons in front of the computer every night, but it’s not necessarily the kind of French he needs.

Rosetta Stone will teach you how to ask for directions to the Centre de Georges Pompidou, but not how to direct play in the white-hot intensity of a match situation.

“I’m grand ordering steak in a restaurant,” he says, “but it’s obviously tough when I want to tell the centres what to do – if I want them to run it a bit shorter, or maybe go wider with the next one.”

Talking is a requirement of his job and something he happens to be very good at. He can be famously stroppy at times. His diary details some of his on-field rows with both teammates and opponents in a comically self-deprecating way, as if possessed at times by some Hydesian alter ego. His account of his history of bad blood with Ronan O’Gara is brilliantly revealing.

When Racing hired Rog to be his kicking coach in Paris, it was as if the gods had decided to make a sitcom out of them – but by then, the two had become firm friends.

It was Brian O’Driscoll who helped them heal the breach. “Brian always said that me and Ronan were very similar,” he says, “and that was why we didn’t get on at the beginning.”

There is a compulsive side to his personality. It is often the way with outhalves. All those hours spent kicking ball after ball through two sticks 5.6metres apart in search of perfection.

“Yeah, I get intense about things,” he says, “and sometimes I wish I wasn’t like that. Even when I’m doing exams or something like that, I work myself up and I have to know absolutely everything.

“I remember around Christmas time last year, when we [Leinster] lost to Clermont in the Heineken Cup. I really wanted us to do three in a row and I was beating myself up about the game for days afterwards. I was down, quiet.

“It was around the time the negotiations weren’t going well and I thought, well, maybe life would be easier if I played for a team that I didn’t care so much about. Maybe it would be different if I wasn’t playing with my best friends – if I was a foreigner, just coming in. So then I arrived here and you throw yourself into it and you find . . . ”

He laughs.

“I want to be a success here,” he says, “so, yeah, you could say that idea didn’t really work out.”

What’s that phrase? No matter where you go, there you are.

He was that way even as a kid. When he was 13, his father – a dyed-in-the-wool Munster man from Listowel – promised him £20 if he could perform 30 keepy-uppies with a football. “It took about six hours,” he says, “but I got the few bob in the end!”

Then he laughs. “I know people are going to twist that and say, ‘God, he was money-hungry even then!’”

He tended to excel at things in which success was quantifiable in numerical terms. Golf. Tennis. Kicking points in rugby. Studying for points in the Leaving Certificate. “What motivates kids?” he asks. “It’s getting approval.”

His mother and father broke up when he was 15. It was about this time, he says in his book, that his friendship with Laura – one of identical twins – hardened into a lifelong love. As the eldest of four children, he probably took the failure of his parents’ marriage harder than his siblings.

“I’d say the younger kids didn’t know what was going on,” he says, “whereas I did. It makes you want to try to get away, to try to focus on anything else. Study. Playing rugby. Playing golf. You submerge yourself in other things.”

He was a rugby prodigy at St Mary’s College. He kicked the winning drop goal in the 2002 Leinster Schools Senior Cup final when he was still in transition year – but he was far from the archetypal southside rugby jock.

“God, no. I had a pretty normal upbringing. Both my parents worked full-time and I had to work in the summer. I think I worked in every shop and business in Rathgar. The Spar. Comans pub. My mum’s shop [the Rathgar Hair Studio]. I used to work Friday nights in Bective Rugby Club when the big European games were on. I’d go out and watch the game, then I’d be back in cleaning glasses.”

He almost didn’t make it as a professional rugby player. You have no idea how close you came to never hearing of Johnny Sexton. As a teenager, he didn’t develop the classic body shape of a player in the pro era. Even now, he has the sloping shoulders of a Burgundy, rather than the hard shoulders of a Bordeaux.

“Usually, the best players get picked for the academy straight out of school. I didn’t, so it was looking bleak for me.”

He had notions of studying medicine, but didn’t get the required points, and wound up doing chemical engineering (“Don’t ask!”) in UCD instead.

He hated it, soon dropped out and found himself, at 19, playing for St Mary’s in the All Ireland League while working for Friends First.

“I started off making coffee and tea and doing small jobs around the office. By the end of it, I was answering the phones to, you know, farmers looking for a loan for a Massey Ferguson or a John Deere, saying, ‘Don’t worry, I’m good for it’!”

As he says himself, he “fluked” his way into his first professional contract.

Mark McDermott, the Ireland U-21 coach, was at Templeville Road one day to watch Gareth Steenson, Dungannon’s fly-half. Johnny scored two tries and landed most of his kicks. He was invited into the academy. After a year, Michael Cheika considered him ready for the Leinster squad – but it still wasn’t happening quickly enough for him.

“I was stroppy and impatient,” he says.

At 21, the age at which Brian O’Driscoll revealed himself to the world with a hat-trick of tries in this same city, Johnny was wondering if it was ever going to happen for him.

“I was always the eighth choice back for Leinster and I always needed one injury to get in.”

When Felipe Contepomi injured himself in the 2009 Heineken Cup semi-final against Munster, he grabbed the opportunity like a man who thought there might not be another. He helped steer Leinster to a first European Cup and put an end to their reputation as inveterate bottlers – and, overnight, acquired a stage name.

“I think it was Michael Cheika at a press conference who first called me Johnny. But I was never Johnny. I was named Jonathan but everyone knew me as Jonno. Like, Laura calls me Jonno. All my old school mates still call me Jonno. My family call me Jonno. But Johnny has kind of stuck. I hated it at the start, but now I sign e-mails and stuff with the name Johnny. I suppose it kind of grew on me.”

As “Johnny” Sexton, all the pieces seemed to fall into place. He took the Ireland number 10 jersey from O’Gara (not without a certain amount of ugliness), played in a World Cup and won two more European Cups, an Amlin Challenge Cup and a Pro 12. Four bountiful years culminated, just as his book does, with him becoming a Lion. And a winning Lion.

It’s difficult for many to separate the series victory over Australia from the decision to drop O’Driscoll for the third and decisive test. He deals with it in the book – “It’s a shock. A huge shock” – but he doesn’t allow it to overshadow the narrative.

“Gatland made the decision,” he says now. “All of Wales thought he was right, all of Ireland thought he was wrong. That’s the politics of the Lions. But you’re very selfish as a professional. I felt for Brian for a day or two days. Then you have to go out and play. As it happens, I got on really well with Johnny Davies, so you can’t be seen to be saying, ‘Ah, this is terrible,’ because you’ve got to go out and play with the guy.

“I remember getting picked for Ireland once and seeing the Munster players sympathising with Rog. I could see them talking to him. And I was going out to play with these guys, thinking, they don’t want to play with me, they want Rog. That can be very unsettling for a player.”

He misses home – and he’s not too proud to admit that he misses Friday and Saturday nights in Ballsbridge.

“And I know some people will say, if you loved Leinster that much, you wouldn’t have left. And if it were someone else, I’d probably be the cynic saying that. But you miss everything about home. You miss the people. You miss, yeah, being recognised.

“It’s very different in Paris. At times, you think, God, I could live here forever. Other times, you think, God, I just want to go home. It’s still in that stage where there’s a lot of ups and downs.

“With foreign players, your motives for being here are being questioned by the French players. All I can do is make sure to work hard and hopefully they’ll see that I’m here for the right reasons and that I’m giving everything.”

He’s never given less than that.