It's Christmas, with all the tinsel and frenzy that brings: the office lunch, the evening parties, those trips to the airport to meet family and friends. Late-night shopping, pantomime openings, overflowing pubs and people thronging to see the latest seasonal movies - endless reasons to be out and about on the streets longer and later than usual.

But there are some people already there who will not be going home at the end of the day. They're the ones who sleep wherever they can find shelter: part of Ireland's increasing homeless population, many of whom are begging.

In Dublin, their homes are doorways in Grafton Street; the garden at the end of South Great George's Street; covered alleys around Government Buildings; the Merchant's Quay archway and other quieter, darker places around the city where a blanket or sleeping bag can be stored in the hope it will be there on returning in the evening.

"I was based out of Dublin from 1990 until earlier this year," says Sgt Fergus Healy of Pearse Street Garda Station, "and what hit me most on my return was the number of people now begging and sleeping on the streets."

At its national conference in October the Simon Community called on the Government "as a matter of urgency, to establish a commission on homelessness which would address the issue in a comprehensive and strategic way". Because, by definition, a homeless population is transient, there are no exact figures for those numbers now sleeping rough.

What is certain is that the Eastern Health Board dealt with 4,000 applications for emergency accommodation in 1996, the majority of those in Dublin. Also last year almost 6,000 people contacted Focus Ireland, which works to alleviate homelessness. The nightly soup run which the Simon community organises year-round to Butt Bridge and Heuston Station also reports a big increase in adults using the service. In the last year the numbers have risen from 53 to 182.

"The people using this service would be sleeping rough," says Donna Doherty, Simon's information officer. The people sleeping rough are those who make up the hard core of the homeless. Other officially homeless people are those who are in emergency accommodation such as voluntarily-run hostels and shelters; those who are accommodated by the Eastern Health Board and those who compose what Focus calls "the hidden homeless" - the ones who move around from friend to friend, sleeping on floors.

Who are these people? Why are they on the streets in such increasing numbers? Neither Focus nor Simon would agree to putting a journalist in contact with people using their services who are on the streets. "It takes our outreach team some time to build up a trusting relationship with young people on the streets. We don't like to jeopardise that trust in any way," says Michael Bruton, chief executive of Focus.

According to Sgt Healy, who has a city-centre beat, the vast majority of those now on the streets are in their late teens and early 20s. Asked why he thinks this is so, he says simply: "You don't last long on the streets."

Sister Stanislaus Kennedy, the president of Focus, says that almost 50 per cent of the people who come to it for help are under the age of 25.

Children begging on the streets, according to Sgt Healy, tend to be mainly from the travelling community and are far fewer in number than the adults. At present, children are less visible on the streets of Dublin, possibly as a result of the ISPCC's new programme which involves approaching children who are begging and returning them to their families. Since ISPCC staff don't wear uniforms they aren't recognised by the children on approach, unlike gardai, who they run away from on sight.

Focus and Simon agree that addiction is a common denominator among adults sleeping rough on the streets, heroin being the primary drug. "It's a long day, a day on the street," points out Sgt Healy. "Drugs help to get someone through the day. And people on the streets tend to think short-term, not long-term. Either someone reaches out and manages to help them or they drift off into the darkness."

With addiction comes a need for money. "The minimum you need to support a heroin habit in Dublin is £30-£40 a day," says Sgt Healy. "Begging is an alternative to crime - but only the fittest survive even at begging. The strongest ones dominate the best pitches." In central Dublin and out as far as Baggot Street and Ranelagh, most cash-dispensing machines and 24-hour shops now have someone begging outside them. Some hold simple cardboard signs saying "Homeless, Please Help". Others hold out paper cups.

"Sometimes you're better off tapping than going out robbing," says Kevin (34), who was sleeping rough in York Street when we met. "Tapping" is the word used to describe begging by people who've done it at one time or another. Kevin left school at 14 and trained as a panel beater. When he was 18 the garage he worked for closed down. He has not been employed since. He was on heroin but now he's on a methadone programme. Over coffee in Bewleys, he is clean-shaven and clear-eyed but looks worn: awake from three o'clock the previous night because of frost.

His main pitch for begging was outside Marks & Spencer on Liffey Street. "The best thing about having a pitch is that you get regular customers." He uses the word "customers" unselfconsciously: that business-like term more usually associated with trade. He knows people begging who collect up to £80 a day on a regular basis. He averaged about £40 a day when he was doing it.

There seems to be little solidarity among the floating population of beggars. "Everyone is out to look after themselves," he says with disquieting honesty.

"Some people go out tapping in pairs although you usually make more on your own." Why is this? "Protection." Kevin has been asked to go out with people - usually women - and stay near them to act as a deterrent. "I'd be minding them from other fellas on the street." He knows people who have been begging and were either beaten up or robbed, or both.

Now selling The Big Issues, he doesn't like sharing a street-space with anyone who is begging or busking. It's competition and, scotching any myth of solidarity among the dispossessed, he adds that he will "have a go" at people who won't move on when he asks. This would be common, especially in busy city-centre locations, such as O'Connell Bridge, Temple Bar, Grafton Street and the streets off it.

Niall Skelly, editor of The Big Issues, says that for the 500 people selling it at any one time it's an alternative to begging. The vendors are either homeless or long-term unemployed. Of the £1.50 cover price, the vendor keeps 80 pence .

"It gives people dignity. The general public are always asking me `Does The Big Issues do any good for the people who sell it?' and I tell them to ask the vendors, not me," says Skelly.

Among the literature which Focus uses to publicise its services are suggested reasons as to why homelessness occurs. "People become homeless because of a breakdown in relationship with a partner or their family and because of lack of income and accessible accommodation." Addiction, marriage breakdown or relationships which have gone wrong are stories Sgt Healy has listened to again and again among the homeless people he talks to on his beat.

Asked where he think the cycle begins - if he thinks being on the street leads to addiction or if addiction results in a life on the street? - he considers carefully: "Somewhere the social system has failed them. It can't support them. But I don't know where that happens."

The next time I meet Kevin, John is with him. He wants to tell how he has ended up begging on the streets. Also 34, John was made redundant from his job with a computer company in 1992. He could not find another job. Depression led to a dependency on sleeping tablets. Then his marriage collapsed and he moved in with friends. "But I couldn't stay there forever."

For a year and a half he slept rough in Setanta Place, begging in Dame Street by day. "Being on the street is like being on a carousel. It goes round and round but you never get anywhere. I used to wake up in the morning with my head full of stuff I didn't want to think about. I wanted to escape and there was nowhere to go except somewhere else in my head." He became addicted to heroin.

"Being on heroin is a 24-hour job," he says. "You have to keep going until you get the money for your fix. I thought of robbing, but I wasn't cut out for it, so I begged instead," he says matter-of-factly. He needed at least £40 a day, eating only "when I remembered. Maybe once every two days." The most he ever made was £250 one bank holiday weekend sitting outside the Olympia, where there was a show on.

"It helps when it rains. People give you more then." He corrects himself. "Women give you more. It was almost always women who gave me money." He often got into physical fights to protect his pitch. "It's everyone for themselves where money is concerned. But there is a bit of support sometimes. People sometimes tell you about a handy place to sleep if they see you out. Somewhere that's dry."

Life on the streets, as told by John, is brutally fragile. Since 1995, when he first slept rough, he has lost six friends who were in a similar situation. John's friends died on the street and in squats. "Cirrhosis of the liver, dirty needles, doing themselves damage when they were out of it, fires they lit to keep warm which went out of control."

The death which has affected him most profoundly is that of a girl who died in a fire last September in a Kevin Street squat with another young girl. "When you're on the streets you never get any good news, but this was the worst." She was one of the people he had previously shared a squat with in Parnell Square.

Having gone through a methadone programme he now sells The Big Issues on O'Connell Street making "about £10-15 a day". He is still officially homeless as he is staying with friends again but still knows many of the people on the streets. Asked to hazard a guess at the numbers presently sleeping rough in Dublin, he replies without hesitation "between 170 and 200": a figure which tallies with recipients of Simon's soup run. How many of these does he think would be heroin addicts? "99 per cent."

I miss Kevin's call from a phone box to say he has with him another friend, a woman who is living on the streets and willing to talk about it there and then. She doesn't make contact again.

Later, Kevin says: "You have to catch us when you can."