"There is never any gravestone (over those who die unnoticed) and I often think that if there was ever going to be a memorial to these chaps - like the Unknown Soldier - a suitable inscription I think, would be `Known Only to God and Alice Leahy'" - Dr Oliver Connolly

"The road is for everybody" - Alice Leahy, aged 7, to a woman who ordered her to get herself and her bicycle off the road

The men and women known only to God and Alice Leahy are not the kind to attract dramatic headlines - even in a week when the ESRI reports that our booming little State boasts more than 3,000 homeless people. Leahy's people are not 13-year-old heroin addicts or mothers of four living in a car. Hers are of a kind that we all knew well, a long time ago. The irony now is that although we don't know them or want to know them anymore, we have never had so much information about them.

"You can get so much information now that it's like wallpaper," says The Irish Times's Padraig O'Morain in a documentary to be shown tomorrow night. "You don't get so much analysis anymore, or anything that's reflective. The danger then is that you take on these ways of seeing things and thinking which can get out of touch with reality."

Sometimes the research becomes the focus, says Alice Leahy. At a recent meeting on mental health, she heard people being talked about as "clients" (as though they had choices), even "inmates".

"You hear professionals talking about `performance indicators' and `best practice' and this is the language of consumerism. And yet if you're not using that language - which is meaningless of course - you will be left out of the debate."

For all her gentle, self-effacing demeanour, Alice Leahy will not be left out of any debate. Her strength is that she knows those of whom she speaks. She is defending no faceless statistics but people she refers to as Johnny and Anthony and Paddy and Frank.

Time was when every town and village in Ireland had a Johnny or an Anthony rambling around the place: someone who didn't quite fit in, who wasn't "the full shilling", who drank too much or was a bit of a loner suffering from that all-encompassing ailment that used to be called "nerves". He might have been a bit "simple" or looked funny at you on a bad day but everyone knew him and he was as much a part of the landscape as the postman or the parish priest. He was given odd jobs of a kind that ensured his inclusion and the occasional dinner in a round of houses where he knew he would be welcome. There was little danger that he would die in a ditch or a derelict house. The fact that he was "different" made him no less a member of his own community. Anyone who has seen the film Cinema Paradiso will recognise him in the role of the village bellringer.

He has been on Alice Leahy's mind for some time now. Twentyfive years ago, above and beyond her duties as a young ward sister busy setting up the first intensive care unit of its kind in Baggot Street , she and Dr David Magee prepared a report entitled Medical Care for the Vagrant in Ireland. The report reflected an Ireland that was already losing touch with its roots. In it, for example, a hostel spokesman remarked that only four or five years before (around 1970), there had been no difficulty getting men admitted to hospital; in the meantime, the position had become "alarming".

The report led to the establishment by Leahy and others of Trust, with ideals no more grandiose than to leave those they befriended feeling a bit better afterwards. Simple things like a shower and a hair wash, a change of clothes, a chiropody session, a chat, a cup of tea, a sense that someone knows that he is Paddy and that he loves football or opera or feeding the ducks but will not judge or seek any more information unless Paddy chooses to talk.

The only thing that's dramatic about Trust's story now, says Leahy, is that their conclusions today would be exactly the same as in that report. "The human condition doesn't change that much," she says. "But we now think that we have become so professional at solving problems that we're forgetting about the simple things that can make life better for people."

Tomorrow night, a documentary made by Anne Daly and Ronan Tynan of Esperanza Productions to be screened by RTE, marks those 25 years of Alice Leahy and Trust, with a sharp and moving reminder of what can happen when a society no longer has place for the outsider.

Daly and Tynan mercifully spare us all the old stereotypes - no cardboard boxes or grimy blankets in city doorways - in favour of engaging with human beings like Frank, Paddy, Anthony, Damien and Moya. They cannot be packaged. Often, there are no easy answers to their homelessness. It might be drink, or gambling or just a sense of always having been different.

Frank firmly attaches all blame to himself. It was the drink, then the gambling. Through them, he lost his wife and adored little boy. "I wake up every morning, sometimes at two or three and I'm asking myself - `why did I do it? How did I let it go, what I had?' " For Paddy, a gentle, white-haired, well-travelled Dubliner, who loves nature and spring time and the mountains where he rambled as a child, drink too played a role: "The drink is a bit upsettin'," he says with some understatement, "it does take over, there's no doubt about it."

Anthony was seven or eight when he was involved in robbing a box of Golly bars (icecream) from a van. The result was that he and his four brothers were placed in care - purportedly because they had no home, which was patently untrue - and he wound up in Artane.

"We were pretty wild," he concedes, "but as a result of being placed into care, it affected all of our lives." Damien, with a sweet, painful smile, says that he has no friends, "not a friend in the world. It's a hard thing to admit . . . I just don't trust people. I started off my life without trust."

God alone and sometimes, Alice Leahy, knows what these people have been through. She has them all in her head and heart: the old man who came back to Ireland after years in London, homeless, still nursing some crippling guilt about an injury caused during a school rugby match in Blackrock College; or the soft-eyed, bearded old man in the painting who tried to pay for a burger with his watch and ended up in the Central Mental Hospital.

It could be depressing. But Leahy's clear-sightedness and the truth shining through this documentary combine to make it more inspirational than anything else. She pays passionate tributes to the many good people who help, quietly and effectively, and they encompass an astonishing breadth of society - churchmen such as Bishop Eamon Walsh, judges such as Michael Moriarty, professors such as James McCormick, singers and poets such as Christy Moore and Micheal O Siadhail, retailers such as Louis Copeland, and visionaries such as Brendan Kenny, the principal housing officer on Dublin Corporation. Her influence runs to the very top. There aren't many who can call on Dublin Castle to launch a television documentary. Alice Leahy managed it - courtesy of Bertie Ahern.

A Fragile City will be broadcast on RTE 1 tomorrow night at 10.10 p.m.