From Castleknock to California, Colin Farrell has travelled a long way. And, internet sex, drug addiction and unplanned fatherhood aside, he's doing very nicely, thanks. As he prepares for the unveiling of his latest movie, Miami Vice, he talks exclusively to Donald Clarke.

SHORTLY after I arrive in Los Angeles for the Miami Vice press junket, a message comes through that I am to be granted an exclusive interview with Colin Farrell. To that point we were under the impression that Farrell, who plays the part originated by Don Johnson in Michael Mann's big-screen updating of his own 1980s cop show, would be communicating with us via a press conference alone.

Colin is, it is true, pretty busy at present. After talking to me he will return to London for a day's shooting on Woody Allen's new project, then immediately zoom back to LA for further Vice commitments.

That noted, before he made himself available to The Irish Times, members of the press were speculating that his people, mindful of the fug of controversy that has surrounded him for the past year or so, might be trying to hide the star away.

Until recently, profiles of Farrell have focused almost exclusively on the actor's dizzying, seemingly unstoppable rise to the top of the Hollywood heap. Born in 1975, raised in the Dublin suburb of Castleknock, Farrell (you hardly need to be told) first gained fame playing Danny Byrne on the BBC series Ballykissangel. In 2000, a year after he left that twinkly show, Joel Schumacher cast him in the Vietnam drama Tigerland. Minority Report, Phone Booth and Daredevil followed.

Colin encountered the odd scandal, but - inheriting, perhaps, an aptitude for fancy footwork from his father, a former Shamrock Rovers player - managed to dance around them all. He is regarded as a hellraiser in Los Angeles, you say. Well, in this town anybody still awake after teatime can gain such a reputation. Yes, he fathered a child out of wedlock. But, acting as other, less responsible stars have not, he never publicly questioned his paternity and has proved an exemplary father.

The first signs that things were going properly awry came with the release of Oliver Stone's Alexander in late 2004. The film, starring a blonde Colin as Alexander the Great, bombed spectacularly with critics before going on to under-perform at the box office. A year later, Farrell was reported to have checked into a treatment facility for an addiction to prescribed painkillers. Then, in January 2006, a tape of him having sex with a former Playmate emerged on the internet. Matters were beginning to look a bit messy.

Strolling down an elevated walkway towards an office in Universal Studios, I find myself fretting about what might have become of our Colin. Will his spell in detox have turned him into a self-regarding, quasi-Californian boob? Will his hitherto admirable tendency towards keeping his problems in perspective have left him? Not a bit of it. Deeply tanned and dazzlingly healthy-looking, Farrell seems as friendly and unpretentious as ever ("How you doing, man? God, this is all mad, isn't it?"), if a tad less sweary.

So, where have we got to now, Colin? How do things stand with Ireland's biggest star, Cillian Murphy still being half-a-length behind?

"It's got a lot more real recently," he says. "It was like I tried this new suit on six years ago and then the suit was crisp and starched. Now it's more relaxed. It fits better on me. Familiarity might breed contempt. But if the novelty ever goes out of it I will knock it right on the head. But, you know, I still can't take it all seriously."

So he can appreciate the ludicrousness of this odd business? "Oh, it's all ridiculous at the end of the day. It was ridiculous sitting on a horse with a head of blonde hair in Alexander. All the stuff that's happened is mad. I remember the Miami Vice TV show when I was eight: the pink flamingos, the Jan Hammer theme song. And all of a sudden I am walking into a bar saying: 'I'm Sonny Crockett.' Mad."

Farrell's tendency, expressed throughout our conversation, to find absurd what others take desperately seriously is a very Irish trait. It must deeply puzzle the movers and shakers he encounters in Hollywood. Indeed, it has struck many domestic observers that the disparity between Irish and Californian attitudes towards recreation has played a significant part in the formation of the Farrell myth.

They see him as an unhinged party animal. We see him as an average Irish bloke with average Irish appetites and average Irish passions.

"I think that's right. We have a laugh. We have a joke. That's just the way we are. I think we have a lot less fear over our station in life and what we own. There is something ingrained in the Irishman to be a provider and that goes back generations. But, yeah, we like a jar. Any Irishman who had been through what I had been through would probably have behaved the same way."

Then he exhales so long and hard he has trouble getting the words out. "But then, at some point recently, I got my arse handed to me. So, you sit back and have a think."

I take it that the reference to arse-handing concerns his medical crisis last December. Shortly after filming on Miami Vice concluded, his publicist confirmed that he had been admitted to a clinic to be treated for an addiction to painkillers (originally prescribed for backache). The internet was, inevitably, quickly awash with rumours to the effect that only a portion of the truth was being revealed.

"I had just been on this trip for a long time," Farrell sighs. "I had been burning the candle at both ends and it beat me down eventually. I was juggling so many balls. I was so concerned with living a normal life. But in reality my life was so abnormal that I didn't have the energy to keep up with working as much as I wanted to work and being the father I wanted to be. So I took a break for a while and, for the first time in six years, got a chance to look at what I'd been through."

Farrell, a Dubliner through and through, displays a terror of seeming too self-regarding, too precious, too up-himself. For every three sentences that touch on the pressures of fame, he delivers one clarifying that he understands how millions will envy him his problems.

"This was the first time I ever really examined myself. Before that I was always: 'Look there's people not working in the world. What's my problem?' I was never up for moaning about it all. I mean, the paparazzi are a pain in the bollocks. But that's a small con with all the pros in my life. This was the first time I was able to sit back and look at what I'd been through and say: 'Jesus. It's been some f***ing trip.' But, you know, I have my son now and that's the best thing in my life."

So the collapse was to do with exhaustion? There were, at the time, mutterings in the media about his ingestion of a wide variety of exotic stimulants.

"Yeah, yeah, yeah," he laughs. "It was mainly just exhaustion. I was worn out to the extent that my body and my mind just wouldn't do it anymore."

The perceived fiasco that was Alexander must surely have contributed to the downturn in his mood. Farrell had featured in critical piñatas before - remember the ghastly SWAT, anyone? - but Stone's film received a truly eye-watering pummelling. The perennially forgiving Roger Ebert identified it as the worst film of the year. Manhola Dargis, writing in the New York Times, mused upon "puerile writing, confused plotting [ and] shockingly off-note performances". The film did eventually make its money back. But the experience has clearly scarred Colin.

"Oh yeah, that was difficult. The film was pummelled and . . . " He pauses and, once again, exhales dramatically. The readers must, it seems, be made aware that he knows how lucky he is. "Look, nothing is the end of anything. I really am somebody who understands the importance of things. My family and friends are the most important thing in my life. But when you put nine or 10 months into something and it becomes the most important thing in your career and then it gets so hammered, it's pretty bad. Oh man, the fucking reviews! They were so personal."

Anthony Lane, the New Yorker's witty critic, displayed more kindness than many when he quipped: "Farrell comes across here as twitchy, straw-haired, and buzzing with sexual mystification, as if he had researched the life of Anne Heche by mistake."

So, Farrell reads reviews? Most actors feign indifference towards the critics. "Oh, God, it was so hard. But, yeah, I read every f***ing one. I almost wrote a bad review myself, I was so into self-loathing."

Still, if newspaper reports are to be believed - and generally they are not - Colin has manoeuvred himself back into a positive place. He admits that the story about him buying a posh house in the Hollywood Hills is accurate, though the mansion pictured in one Irish paper was not his. "Colin Farrell has bought a house in the same city as this house," he jokes. The actor wanted to help his two-year-old son, who lives in LA with his mother, model Kim Bordenave, gain some sense of stability and has, accordingly, set aside one of the rooms for young James.

What does he do with his evenings when staying in Hollywood? I can't really imagine him fitting in with the air-kissers.

"I don't really do it, man. I mean, in the first days when I was here and I was running amok I never really went to the right places. You know, there's a particular club you go to on Tuesday, a club you go to on Wednesday. I never got into that. When we went to Miami for Miami Vice, I didn't hang out in South Beach either. I went to this little bar called Tobacco Road. I was there every day. At the end, they gave me my barstool I was there so often."

Some of the reports on Farrell's new house, noting one photograph in which he appeared to be wearing a ring, suggested that he might have secretly married his then girlfriend, actor Lake Bell. "I believe so," he laughs. "You see how little I think of myself. I didn't even invite myself to my own wedding. I wasn't there. That's how little I think of myself."

He has just finished shooting Pride and Glory, a thriller co-starring Ed Norton, and is currently filming with Ewan McGregor on the evocatively named Untitled Woody Allen Summer Project. Neither of those films will reach us until 2007. For now, we can enjoy Farrell and Jamie Foxx, angry undercover detectives both, inconveniencing savage drug traffickers in the strange entity that is Miami Vice. A kind of anti-Starsky & Hutch, the picture, rather than drifting into pastiche, proves to be immeasurably grimmer than its small- screen predecessor.

Reading the stories about the hugely troublesome shoot - hurricanes, local crime and other logistic convulsions caused significant overruns - one might reasonably jump to the conclusion that Mann's darkly stylish film has soaked up some of the chaos that surrounded its making. At one stage, while shooting in the Dominican Republic, a gunfight took place outside the film-makers' compound.

"Oh god, yeah," Farrell says. "That was something. I had worked out on the gun range, so I knew the sound of one when I heard it. We just bolted to the corner of the room and closed the door. But I wasn't that worried. Actually, that's a fib. When the gunshot went off I did think for a moment: maybe there are militiamen out there or whatever. As it happened it was just a drunken policeman who wanted to get on the set."

It's hard to know what the public will make of Miami Vice. Neither as suave as earlier Mann films such as Heat nor as ribald as the average TV adaptation, the picture may prove a hard sell. But Farrell remains philosophical about his prospects. If it all ended tomorrow, I suspect he would take it passably well.

"I feel like every film I work on is a new experience and opens up new challenges," he says, when asked about his unfulfilled ambitions. "I wouldn't mind writing or directing something. I have a few ideas for something small and personal. It would be great to go back home and give some work to all the drivers and crew and actors from the Ballykissangel days. Myself and Jim Sheridan are trying hard to do something we can shoot back home."

Before we close, I just have to throw one particular piece of tabloid gossip at him. "Ah, yeah, go ahead, man," he says cheerily, before arranging his face to shrug off inquiries about the sex tape.

Never mind that. What about the story, reported by the actress herself, that he spent an entire evening trying to seduce the septuagenarian Dame Eileen Atkins, his co-star on this year's underrated Ask the Dust? I suspect the yarn was a gag the two of them dreamt up.

"Oh right. Yeah, that," he says, visibly surprised (relieved?) that I've focused on this. He adopts a mock-pompous tone. "I can neither confirm nor deny the Eileen Atkins story. That is between myself and the Dame."

It must be a pain for readers to plough through endless profiles in which journalists fail to say anything properly nasty about Colin Farrell.

But the truth is he is an interviewer's delight. Even when, as above, he refuses to answer a question, he does so with admirable good humour. But the strain of remaining civil when the media is packed with lies about your private life is surely exhausting. A few days after we met, Farrell was accosted by a woman who leapt out of the audience while he was talking on The Jay Leno Show.

The woman, Dessarae Bradford, had unsuccessfully alleged in a small-claims lawsuit that Farrell stalked her with inappropriate calls and text messages. According to a member of the audience, Bradford shouted "I'll see you in court," and Farrell replied "you're insane", before she was hustled away by security staff. He must, from time to time, regret going into the business.

"Oh, no. It's my life and I am really lucky. I continue to count my blessings. I no longer pinch myself, because I know this is now my life. When I finally got that chance to look at it from the outside I realised it's not normal. But, then, I don't know what a normal life is."

So he never wishes that he could have achieved his original ambition and become a footballer like his dad?

"Oh, yeah, well that's different," he laughs. "If I could do that I would do it in a second. In a second. If I could put on the green shirt and represent Ireland, I'd change it all for that tomorrow."

Miami Vice opens next Friday