He has been described as "a loose, saggy giant" with a "crumpled paper bag of a face". Not exactly complimentary: but then, if you've been dubbed "The Great Gambon" by Ralph Richardson on account of your performance in a Brecht play, not to mention invited to "Arise, Sir Michael" by Queen Elizabeth on account of your extraordinary contribution to the arts, who needs compliments? In any case, as Michael Gambon crosses the lobby of the Westbury Hotel, six foot two in a creamy-coloured jacket, the face - pale and somewhat weary-looking, certainly, as befits a rainy summer Saturday in Dublin - arranges itself into a beaming smile. "I'm back, after all these years," he declares. "It's a nice feeling, I must say. It's like a big circle."

It turns out to be an uncharacteristically upbeat statement from a man who can, in a single sentence, describe the plot of a recent smash hit on Broadway as "it all ends in disaster; a laugh a minute", refer to the interpretation of Ben Johnson which had the British theatre critics in raptures as "doing Volpone" and conclude, without pausing for breath, "so, touch wood, I've been doing all right". Who sums up a glittering career with a lugubrious - "I just plod on. It all flies by. You don't really know how it happens" - and professes to be somewhat dazed by his own productivity. "God, it's amazing, isn't it, when you talk about what you've been doing, one thing after another like this."

You should see your biog, I tell him. "What, long, is it?" Terrifying. "Jesus Christ," he exclaims, and looks genuinely shocked. Gambon is in town to play Captain Boyle in Garry Hynes's forthcoming production of Juno and the Paycock at the Gaiety Theatre, his first paid role on a Dublin stage since he wangled his way into a job with Micheal MacLiammoir and Hilton Edwards at the Gate in 1962 - which was his first paid role of any kind. Impressed by, or indifferent to, his claim to have played the lead in Shaw's Candida in the West End and his offer to stop off in Dublin for an audition "on his way to Broadway to see about a play", MacLiammoir and Edwards gave him a bit part and took him on a tour of Europe in a cast that also, he remembers, featured the actor and writer Christopher Fitzsimons.

"I told a load of lies, but they didn't know, did they? They just needed bodies." When he joined Olivier's National Theatre in London the following year, it was - literally - to stand on stage holding a spear. "That was the way theatre was in those days. Nobody does it like that any more - they couldn't afford it. I stayed there for three or four years, gradually got bigger parts. The old, well-worn route."

This is a return of another kind, too, for Gambon was born in Dublin, the son of an army officer and a jeweller's daughter who moved to London at the end of the second World War. "Like so many others, we went to Camden Town because that was where the Holyhead train came in, at Euston Station. My dad said we lived as far away from the station as he could carry the suitcases. Simple as that." So is he an Irish actor, or an English actor? "Well, I was brought up in London, so I suppose I've become English, haven't I? It's confusing, who you are, isn't it? When I was growing up my mother and father spoke Dublin, so I'd speak Dublin to them at home," he says, in an accent you could sell on Moore Street. "And then you'd walk out the front door and you were in London, weren't you, with your friends, and you'd talk" - he switches to brittle Cockney - "like that".

The Camden upbringing was anything but confusing, however. "I was a very quiet boy. Went to a Catholic school - St Aloysius - and was very happy there. Played the cello. Did the usual things that kids do. When you go into that environment you don't leave Ireland at all, in a way." But the wider world reached in to Camden in a way it never would have in 1950s Phibsboro.

"We had the editor of a London black newspaper living down below who had African tribal art all over the walls of his flat. To a six-year-old who had just come in from Dublin, that was a magical place." His early experiences in the theatre, by contrast, were plebeian to the core; around the corner from his home was an amateur theatre run by the Communist party.

"It was called the Unity Theatre, set up after the war to present socialist drama on a part-time basis. I used to help build sets, and then they gave me a few acting parts. It was run by Bill Owen, who was in Last of the Summer Wine. So that's how I started. You start doing it part-time, and then you suddenly realise you can make a living doing this."

Making a living is one thing: becoming a household name is quite another. The terrifying biog lists Shakespeare (Richard III, Othello, Macbeth, Coriolanus, King Lear) alongside Alan Ayckbourn, Simon Gray and David Hare, and is sprinkled with premieres, leads, title roles and awards. But people in the street don't, by and large, know that: when they recognise Michael Gambon, it is for his role as Philip E. Marlow in the 1986 television series The Singing Detective.

Directed by Jon Amiel with a screenplay by Dennis Potter and a glorious big-band soundtrack which was retro before retro was invented, the series is still, more than a decade later, spoken of in hushed tones as a classic of television. How did Gambon get involved in it? "God knows. I was just sent the script by my agent. Went to eat pasta with the director in a restaurant. He said `will you?' I said `yeah'."

But even as they were shooting it, they knew they were making something special. "It took seven months to do - three months playing the fellow with the coat and hat in the street, and four months lying in that bed covered in psoriasis. It was a hard job, but it was great. I fear they'll never make things like that again: television has changed so much they wouldn't have the money to do it now - or rather, they wouldn't invest the money in drama like that. Cooking. That's what they make; cookery programmes. It's bloody awful what's happened to television. I don't know who'll be making The Singing Detective, or its equivalent, in 20 years' time."

The regret in his voice seems real enough, as does his affection for the wayward genius Dennis Potter. "He was at the first-ever read-through, sitting opposite me, and I said `I'm frightened of you'. And he said `why?' I said, `you're an intellectual writer. I'm just an ordinary actor. I've nothing to say to you'. He said, `oh, God, that's terrible - what are we going to do? What are you interested in?' I said, `well, I'm quite interested in cars'. So whenever he came on the set, instead of talking about The Singing Detective or the writing or whatever, he'd say, `have you seen the new Ford whatever . . . ?' My relationship with him was one of jokes. Nice man."

He applies precisely the same phrase to John Banville, whose script for Deborah Warner's film of Elizabeth Bowen's novel The Last September, due to open in Dublin next month, impressed him greatly. Of the film itself he says, "I don't know what it's like. I hope it's good. And then I did Lughnasa before that, didn't I, which was quite successful here, but I don't think it did very well elsewhere. That was a dream job - wonderful actors, the director was fun, it was two months of bliss. I've never enjoyed a job so much."

When Dancing at Lughnasa came out on video, however, the reviewers closed in like vampires. While reaction may have been "mixed", to use the polite term, at least one critic pronounced Gambon's performance "awful" - which doesn't surprise him in the least. "I've never been in a success, you see. I spent five months in Hollywood making a film with Barry Levinson. It was called Toys, with Robin Williams. It wasn't a good film - but still, you know, five months in the swim of Hollywood film-making, turning up at 20th-Century Fox every day and being treated like a lord, flash cars and a lot of money. I thought my phone would never stop ringing after that. And it came out on the Tuesday and that was the end."

How does he manage the balance between theatre and film? "Just by luck, really; I suppose I'm essentially a theatre actor. Television has dried up for everyone in the last few years. I used to do bags of it, but films have almost taken its place in terms of work. Although I've just finished playing the clock-maker John Harrison in Longitude, which was a mammoth thing, a two-part television film for Granada Films."

Before that, he did Mrs Gaskell's Wives and Daughters and before that again, a film with Tim Burton and Johnny Depp called Sleepy Hollow. "So from last November until just recently I've been wearing the same costume - breeches, stockings, high-heeled shoes and a big hat. And you forget which one you're in, because on a Friday you're standing in front of a camera doing Mrs Gaskell's drama about a family in northern England in a stately home, and on Monday you're doing Longitude about John Harrison in a stately home in London. And the accent is different but the camera is the same make, isn't it? "My God, I've just finished Longitude and I forgot all about it until just now. I go from one job to another, and you fall in love with people, you get fond of people and there are tearful goodbyes, but I even forgot Longitude - 200 people just go, whoosh, out of your brain. And now I'm here and the whole process starts again. It's not good for you. I think it makes you miserable."

Is he? Miserable? "I'm quite sad sometimes, and that must be the reason, that I get to know people and then don't see them again. I'm just quite maudlin at the moment, because of Longitude, I think."

It's hard to tell how much of this ostentatious gloominess is for real, and how much is a smokescreen Gambon pulls over himself for the duration of an interview. He avoids publicity when he can, and plays puckish games when he can't, once "confessing" to a hapless hack that he was gay, just for the hell of it. He is often quoted as saying that an actor should keep his personality - if he has one - to himself. But surely even he won't be able to resist a bit of gossip about his recent knighthood? He can't. Describing the arrival of the photocopied letter from the prime minister's office, he actually allows himself to get quite chuckly.

`It says `Dear . . . ' and then your name filled in with a pen, `the Prime Minister, blah, blah, if you'd accept . . . ' And then a gap with `knighthood' filled in. And it goes on `If you accept, please tear off the strip below . . . ', and there's a serrated bit and a box to tick. And a prepaid envelope. And then three months later you go and queue up at Buckingham Palace and get your medal. I had a smaller one before, a CBE, and now I've got a bigger one. It's very nice, isn't it, as long as you don't take it too seriously. It's a bit of fun, that's all. Ian McKellen rang me up the day after I got it and said, `I'm giving mine back if they're giving you one - it devalues mine so badly, I'm giving it back'."

The memory of McKellen's mischievous call has cheered Gambon up so much that he offers another cheery story, this time a propos of his interest in flying single-engine planes. "That's good fun, too - though I'm not a very adventurous pilot. I'm very safe, you know. I have to do so many hours a year to keep the licence up. I went to LA by Virgin Atlantic six months ago, and over Greenland or somewhere, at 30,000 feet up, I was on the flight deck with the crew. And they're all bored stiff, aren't they, with the automatics on. And the captain said, `let's play a game where we both die, and you have to get this Jumbo safely down, and all you can fly is a little single-engine aeroplane'. So I said, `well, how are we gonna do it?' He said, `You just tell me everything you'd do - which book you'd take out of this little compartment here to look up, how would you turn the aircraft round, what buttons would you press'. And we had the greatest two hours ever. They were in hysterics laughing at me - but eventually they agreed, the captain and the first officer, that I would have got it down on to the runway. Whether or not I would have burst the tyres, they weren't sure. But I was quite pleased about that."

We're on a roll. He's beaming again, the misery of the actor's lot temporarily forgotten: time to ask what, if anything, he'd still like to do.

"I'd like to play King Lear again. I did it in Stratford in 1982. And I'd like to play Falstaff parts one and two. They're the only things I'd like to have a go at again; they're both very demanding of great energy and breath, physicality, so I suppose you've got to do them before you get too decrepit. Well, I'm not that decrepit now - but you've gotta get them in quick."

What's it like, returning to Shakespeare after a diet of films and television drama? "It's like a breath of fresh air, really, because it supports you, doesn't it, good writing. Like O'Casey. If you obey what O'Casey writes, and you learn it properly - all the full stops and commas and semi-colons - you can just sit on it. It makes acting easier. It sounds a bit pompous, but it's true. I'm not saying you don't have to act; but it certainly holds you there.

"I've done a lot of Ayckbourn, and I've got to know him quite well, and some of his plays are wonderful, but they constrict you rather - some modern writing traps you in a framework that you can't get out of. Ayckbourn in particular is like being in a machine; it's very well put together, but if you start fooling around with it, well . . . with Shakespeare and O'Casey you can fly up on them, and you can take risks. With new plays, too, you often find yourself adding in another sentence because you have to get, say, from this side of the stage to that one to answer the phone and there's no dialogue to cover it. So you're adding bits all the time. And the writer sits there nodding; you know he's quite happy for you to do that. And then you begin to lose respect for the play. You think `hey'. I've had that a few times."

Gambon pauses before delivering the killer punch, timing it, as you would expect, to perfection. "You feel," he says, the face adopting an expression somewhere between wry and coy, "as if you're not sitting on a gem."

Juno and the Paycock opens at the Gaiety Theatre, Dublin, on September 9th, with previews from September 2nd