He might be a drunk and a bum but he still has that most precious of musical things - a unique and special legacy

On Christmas Day, God willing, Shane MacGowan will confound many of his friends and fans - not to mention a legion of obituarists - by marking his 50th birthday. The half century is a miracle for a singer and songwriter who, despite measuring out his life with spirits and acid tabs, has still managed to front, get kicked out of and rejoin The Pogues, co-write pop's most poignant secular Christmas carol and even keep the odd tooth in his head.

It's hard to know whether MacGowan is better known today for Fairytale of New York, the filthy, tender duet he and the late Kirsty MacColl first belted out in 1987, or for his legendary thirst. The song, which he penned with fellow Pogue Jem Finer, returns like the debauched ghost of Christmas past to haunt pubs and clubs each December. But it has enjoyed more than its usual share of airplay and discussion this week after Radio 1 decided to bleep out the word faggot from its lyrics, only to rethink the decision hours later. "It was a bit daft, wasn't it?" says Gerry O'Boyle, who owns the Boogaloo pub in north London, where MacGowan is often to be found. "There was no need for it; it's been out for 20 years."

According to Conor McNicholas, editor of NME, the song deserves its place in history. "The world is a better place for Fairytale Of New York." It is, he says, "a moment when pop music becomes a real work of art - it's as much a short musical drama as it is a pop song".

MacGowan's other claim to fame springs from his childhood in north-west Tipperary. He says his parents gave him Guinness and whiskey as a child because they believed those who are denied drink when young turn into alcoholic adults.

His family moved to London when he was six, and by the time he was a pupil at Westminster, one of the country's oldest and most prestigious public schools, he had discovered other pleasures. MacGowan's academic career in the shadow of the abbey lasted a few months before he was kicked out for taking drugs.

The London punk scene of the late 70s coaxed MacGowan away from a string of temporary jobs, and led first to the formation of the punk band The Nipple Erectors and then The Pogues, who fused punk with traditional Irish music.

Peter "Spider" Stacy, one of the founders of the Pogues, first met MacGowan at the urinal of the Roundhouse when The Ramones played there in 1977. "He's a very strong character and he makes a very strong impression on people," he says. "But I took to him straight away and was impressed by his intelligence and erudition; he's very well read."

The Pogues recorded five albums together before MacGowan was sacked in 1991 because of his drinking. "Shane is a genuine songwriting genius, but he can also be a complete bastard," says McNicholas. "A friend of mine was drinking in [a bar] one night many years ago when Shane was there.

"As the evening drew to a close Shane had managed to piss off the whole pub with his behaviour. He demanded another gin and tonic so the barmaid dropped in a slug of gin and then got everyone in the bar to spit in the glass before she stirred it up and served Shane the foamy mess. Paralytic, he declared it 'the best gin and tonic I've ever had'. I always took that as the measure of the man as an individual."

Over the next decade he immersed himself in a cocktail of wine, gin and tonic, long island iced tea, port and martini and put together a new band, Shane MacGowan and the Popes. He collaborated with The Jesus and Mary Chain and Nick Cave - and continued to take drugs.

His girlfriend, Victoria Mary Clarke, was once called to his house to find blood gushing from his mouth after he had tried to eat volume three of The Beach Boys' greatest hits.

"[Shane] had become convinced that the third world war was taking place and that he, as the leader of the Irish republic, was holding a summit meeting in his kitchen between the heads of state of the world superpowers, Russia, China, America and Ireland," she wrote in the Guardian. "In order to demonstrate the cultural inferiority of the United States, he was eating a Beach Boys album."

In 2001 MacGowan rejoined the Pogues and the band continues to tour to this day. Several members have stopped drinking, but one has not. "Shane is Shane and he does what he wants, but he is essentially a fairly calm person, contrary to impressions," says Stacy. "Like anyone who drinks, sometimes he drinks too much ... [but] he's older and wiser now, that's the cliche."

O'Boyle agrees. "Shane is just Shane and he's a very down-to-earth person; it's not his style to show off. He is a very unique talent and a gentle soul."

MacGowan himself, however, is well aware of the mythology that envelops him. "In Irish pubs where they still sing, Fairytale has become as much a standard as Danny Boy or The Fields of Athenry," he wrote on a Guardian blog last Christmas. "So I'm like the writers of all those traditional standards, except I'm not anonymous. Or dead."

And despite the drink and the drugs, the fall-outs and the punch-ups, MacGowan's music looks likely to endure.

"He might be a drunk and a bum but Shane MacGowan still has that most precious of musical things - a unique and special legacy," says McNicholas. "With that in your top pocket you can drink yourself off your bar stool every night as far as I'm concerned."

The CV

Born December 25 1957, Kent, but raised in County Tipperary

Education scholarship to Westminster School, but expelled for drug use

Bands 1977 - 1980 The Nipple Erectors; 1982 - 1991 The Pogues (changed from Pogue Mahone, Gaelic for "kiss my arse", after complaints); 1994 Shane MacGowan and the Popes. Pogues reunion gigs from 2001

Biggest commercial success Reaching No 2 in UK with Kirsty MacColl duet, Fairytale of New York, in 1987