"I slipped on the bathroom floor going to bed and crushed a vertebrae, " he says. "I don't know how when Laurel and Hardy fell down they never hurt themselves. It's one of those things that has to cure itself. It feels like a dead weight hanging off one's shoulders. It's a bloody nuisance. I've had to cancel holidays."

It was months before it was diagnosed. "I went to my GP because I wanted to get a signature for my bus pass. I ended up with a week in hospital and a man's hand up my backside.

I woke from a local anaesthetic and was told I had an ulcer and a touch of cirrhosis and a spinal problem."

His pauses and sips from a glass. "I'm supposed to drink 12 glasses of liquid a day. Once in a while I feel I could use a drink.

But I've been told not to so I don't."

Not like when he was recovering from a triple heart bypass 12 years ago. He was told he could only have two drinks a day.

He used up his quota in the bar at the Abbey during the interval for his double bill, Chamber Music.

When the bell rang he noticed someone had left an unfinished whiskey on the counter. He drank it and told me, "It doesn't count, it wasn't mine."

He has always toyed around with time and memory in his plays to reveal unexpected personal truths. In Da, the play that consolidated his international reputation on Broadway by winning a Tony Award, the past has the same immediacy as the present. His fictionalised older self returns as a successful writer from England to confront his younger self at his father's funeral. In Love In The Title three young women talking together turn out to be a grandmother in 1932, a mother in 1964 and her daughter in 1999, while there is a 30-year gap between Act One and Act Two in Moving, yet the characters observed in each act moving into a new house do not age in the slightest. They remain the same people at the same age.

For Leonard life is a continuing interplay of now and then, a past that stubbornly won't go away and is always present. "The trouble now is that the far past is more clear than the recent past, " he says.

After 20 plays, not to mention films, television adaptations and series, he claims he has now written his last. It's called Magicality and is about the old theatrical fit-ups.

"It's a skit on Pinteresque drama, the long pauses, " he says.

"People think highly of it but nobody wants to do it. The Abbey say they want a play about the new Ireland. They don't seem to realise that the new can be in the old."

He has given up his longrunning curmudgeon's column in the Sunday Independent too. "I don't fancy going back to it, " he says. "The deadline is a bit of a strain as you get old. I find it easier to write a book. There's no collaboration involved. That's a great bonus, not to have a director and cast. I love actors, but not necessarily in my plays."

He's nearly finished writing The Devil For Grandeur which, with Home Before Dark and Out After Dark, will complete his autobiography. "I've just a chapter to go, " he says. "It sums up my mother. I wrote about half of it and then I said, wait a minute, I've written a play about my father, it's high time my mother got a look in. So I'll write a book where she was the devil for grandeur. She always wore her best hat. She was a great one for the social graces thing."

He never knew his real mother, Annie Byrne, who lived in lodging rooms on the North Circular Road and gave him up for adoption when he was 10 days old. He was reared in a twobedroom cottage on Kalafat lane in Dalkey by Mag Keyes, whose husband was a gardener for the Bewley family. On his birth certificate his name was John Byrne, but in Dalkey he was Jack Keyes. His real self, as he says in Home After Dark, "was somewhere between both names, for he had no wish to be the first and no right to be the second".

To compound the confusion, he then assumed the pen-name Hugh Leonard when his first play, The Big Birthday, was staged at the Abbey in 1956. He was afraid that if it was known he was doubling as a playwright he'd lose his job as a clerk with the Land Commission. "I must have been crackers, " he says.

Hugh Leonard became a public face that allowed him to have a private life. "The Hugh Leonard name is something I don't answer to. You put it on the title page of a script. That's all it's good for.

Anyone who knows me as Hugh Leonard doesn't know me. Jack Leonard is the name I'm probably more used too.

"My poor mother wouldn't know a pen-name from a hole in the wall. She wouldn't know what was going on. I remember once this woman came to me and said, your mother was disgusted by that thing you wrote. What thing?

The Sinners. She kept on saying disgusted as if it was a kind of mantra. I realised she'd just been down to the house to see my mother, saying you ought to be ashamed of him, and my mother not knowing where to look or what I had done to make her ashamed. She didn't know what I was, she didn't know what stuff I was writing. You get things like that. There's a kind of underworld in Dalkey which is sort of a small microcosm of Ireland. I sort of feel I should have packed up the cats long ago and gone somewhere where it was warm and where you live."

When he returned to Ireland in 1970 with his wife Paule, the daughter of a Belgian diplomat, and 13-year-old daughter Danielle, attracted by the Charlie Haughey tax break for writers, he had no particular desire to live in Dalkey.

"We wanted somewhere in the bay, maybe Howth. After living for a while in Killiney Heath . . .

otherwise known as Disneyland . . .

and then at Coliemore Harbour we eventually found this apartment. Paule said I wouldn't mind that. She never said I want.

So I got in touch with my accountant Russell Murphy. It was his last act for us. He was about ready to fall in the grave.

Once it was known I was interested, the price went up.

Russell said the place is no longer viable. Paule said we're going to lose it. So I called back Russell.

Paule wants the place and I want her to have it. He got it and he rang Paule, don't worry my dear, you will have it, bricks and mortar, bricks and mortar. You do trust me, don't you? Those were his last words."

After Murphy's death it turned out that £250,000 of Leonard's money was missing. "Isn't it great that I had a quarter of a million that I could afford to lose?" he was able to joke to me at the time.

Humour has always been his release and also his defence. He has never been able to resist a cutting line, even if it meant losing a friend. Bullied as a scholarship boy at Presentation College, Glasthule, he "learned to armour myself by cultivating a skill at deadly insult". He is easily hurt. "People who feel rejected may spend their lives looking for acceptance, " he once told me.

Paule died six years ago. "There by the door onto the veranda, trying to get some air while we were waiting for the paramedics, " he says. "She was happy here.

There was a good feel to it."

He met Paule while he was acting in plays in the Land Commission drama group. "I was quite good at comedy, but give me All My Sons playing the lead and I was hopeless. It was great training. You learn about painting sets, you learn a bit about production, you learn to memorise a play. I could only remember lines in a bus or on a tram going into town. That's where I'd look at a script and after three or four trips I'd know my lines."

He'd grown up on Hollywood cinema. After school every Monday and Friday he'd take the tram to Dun Laoghaire, get out at the Picture House and his mother would be there in the front row of the middle stalls. "She'd have a packet of sandwiches and a Baby Power with milk, my lunch."

He applied the craftsmanship of films like Lost Horizon, King's Row and Capra's It's A Wonderful Life to his plays. There is an admirable logic to their structure.

Nothing is wasted. The comic timing is flawlessly calculated.

"The one thing movies did give you was a sense of form about your work, " he told me when we first met, attacking censorship on the Late Late Show in 1971.

He will be 80 on Thursday. To celebrate the occasion the Dalkey Players, directed by his friend Margaret Dunne, are staging one of his funniest plays, Time Was.

"The idea began as a spoof for Ronnie Barker about two middleaged husbands imagining they were Laurel and Hardy, " he says.

He no longer keeps a diary. "I found out that Paule was reading it, so I stopped. I'm sorry in a way, but there was no place I could hide it. The last entry I did was the day Paule died. I didn't want to do it any more. We had our fights. But we sort of became reconciled not long before she died."

He regularly has lunch with Bernard Farrell, the playwright with whom he has most in common. He once joked that they were the only good Irish playwrights, all the others were geniuses.

He and his American partner, Kathy Hayes, go everywhere together. "I don't know how I got Kathy, " he says. "I found her in a game of Trivial Pursuit in Cape Town. I don't know what in the world persuaded her to come here. I think she saw Da on the stage with Barnard Hughes . . . that may have done it. She now knows more people in Dalkey than I do.

She'll talk to anybody. But me, I don't take the trouble and as a result I'm regarded generally as surly, which I'm not."

They're planning to go to the south of France with his daughter Danielle, who has moved back from London to live with them, when his back improves. He can no longer manage the barges he loved to hire every year to explore the French canals. "I can't throw a rope and it really needs two hands to handle the boat."

But first he has to complete the last chapter of his book. "And then I think I'll just retire. Or sit down and die. Something like that. Give myself a rest.

Whatever."