Jasmine Guinness belongs to one of Ireland's most famous - and richest - families. She grew up in the grounds of a 12th-century castle where her grandparents and parents entertained rock'n'roll royalty, including the Rolling Stones. Less happily, Diana Mitford, wife of the British fascist leader Oswald Mosley and a keen supporter of Hitler, was her great-grandmother.

But her father Patrick Guinness's dynastic clan of old money and bohemia is a world apart from her mother Liz Casey's family of persecuted and impoverished farmers.

She has always been intrigued by the mystery surrounding her maternal forebears. Last year she began to look into the past as part of a family history show on Irish television. She discovered that while her father's family made vast fortunes during the famine years, the Caseys virtually starved.

"People tend to assume that I come from a long line of castle-dwelling gentry and am made of money," says the 32-year-old model when we meet in her toyshop, Honeyjam, on Portobello Road in Notting Hill. "If I meet someone with a name like Rothschild or Astor, I fall for that, too, and immediately think they're minted. So I don't blame people for making assumptions. The programme made me realise what a huge mix we all are."

There were no noted names in her maternal lineage. The Caseys, displaced Catholic refugees from Belfast, endured mob prejudice and terrible poverty. They fled to Cork and eked out a living in the deprived, disease-ridden rural community of 19th-century Tyrone.

"I knew nothing about my mum's family," she says. "Her parents were dead by the time she was 14. She was brought up by two aunts and she only ever met one uncle."

Guinness lives in London with her producer husband Gawain Rainey, 38, and their two children Elwood, seven, and Otis, three. She travelled with her mother to the island farm in Tyrone to seek out the ghosts of her little-known relatives.

She came close to tears when they visited the marshy land, contemplating the struggles of her mother's great-great-grandparents, Joseph and Maeve Casey, and their son, Francis, as starvation raged through the county. "It's an amazing feeling standing exactly where your family stood centuries ago, looking at the view that they saw every day and feeling the earth that they worked every day under your feet."

Guinness has long been obsessed, she says, with the whims of fate: how small choices, chance encounters, and strokes of fortune can alter the course of a life or determine our existence."If my mum's great-great-grandparents hadn't sent off their only son because they couldn't feed him, they would have perished. As they were flax farmers, he went to an apprenticeship to learn about manufacturing oil-cloth and tarpaulin. We discovered that he married a prosperous farmer's daughter from Moy and became a successful businessman making and selling oil-cloth and tarpaulin to ship-builders in Belfast. I felt thankful to them for the hard decisions they made."

She found the plight of her long-lost ancestors humbling and was impressed by how fortunate they were to have survived when millions of others died. "It helps you appreciate the huge sacrifices that people have made along the way. Their landlord, the Earl of Caledon, set up soup kitchens just to keep his tenants alive and the only reason they survived is because of the kindness of this one man."

She also has a strong sense of pride: "I was pleased that my ancestors were hard-working, Gaelic-speaking Catholics. It wasn't an easy thing to be at that time, so I know they had strong beliefs and were brave in the face of adversity.

"Mum and I were delighted to find out we were descended from 'bog-trotters'. In the programme, we were laughing about us really being 'bog-trotters' because here we are standing in a bog and this is where we're from."

But when the programme was broadcast in Ireland, Guinness got a lot of flak. "The papers were really snotty," she says with a giggle, plainly not unduly hurt. "They took the piss and wrote things like: 'Oh, poor Jasmine Guinness. How dare she say that she came from peasant farmers in Tyrone when they had 22 acres?' But what they failed to mention is that my mum's family were tenant farmers. These weren't our 22 acres. They had nothing."

In fact, her sympathy is for more than just her forebears. "It was more of a shock thinking about how the famine affected everyone. How many people died by the side of the road, in the streets of Dublin or in a hellish coffin ship and eventually watery graves as they tried to emigrate?"

The activities of her paternal family gave little comfort. They controlled much of the country's barley harvest to brew velvety Irish stout for the English market, which, along with meat, barley and wheat exports, was shipped under armed guard to Britain as the starving multitudes looked on in despair. They also exploited the desperation of hard-up families who were fleeing the country by buying up disused and abandoned land at rock-bottom prices to set up their family estates.

Yet Guinness refuses to condemn her millionaire ancestors. "The family did contribute to famine-relief funds but that was edited out of the show. Guinness was the first to introduce pensions for its workers 200 years ago. It built schools, hospitals and housing, all of which was unheard of at that time."

Being a Guinness, she concedes, is a double-edged sword. "I'm very proud of the family name. It can be really helpful sometimes and I'm happy to use it shamelessly if I'm asking favours for a good cause. Butwhen I started modelling there was a lot of resentment."

She also has the sprawling Mitford legacy to contend with and the long shadow of her great-grandmother Diana, from whom she has inherited her ice-blue eyes. "I only met her a few times and thought she was scary. As for the fascism, it was awful, appalling ... really stupid. I suppose you fall in love with a man, you support him until the end, so to a small extent I understand that. But I don't agree with her politics."

Despite her surname and glamorous background, Guinness insists that she is not another couture-loving socialite but works to earn her crust: "As it happens, I grew up on a beautiful farm and my grandparents lived in this incredibly grand house next door. But I haven't been brought up with the mentality of an heiress," she explains. "The only reason my parents got by is because my grandfather gave us a cottage to live in."

That gilded world of inherited wealth is long gone: "I've worked since I was 18. I have no trust fund and don't expect anything. The truth is, there are so many of us now that there isn't enough money to go around," she says.

The toyshop, which she co-owns with a childhood friend, is her day job. And her children, she says, put the work into perspective. "It may sound cliched, but when you become a parent suddenly nothing else is important any longer."

Her own childhood was played out at her grandfather Desmond Guinness's Leixlip Castle, with bonfires under the stars, ponies and blues records. She shared a little guest cottage with her parents amid the communal, hippyish atmosphere of an extended, chaotic family and friends. "My mother was only 17 when I was born, so the three of us grew up together in a way."

Surrounded by a perpetual rock'n'roll circus, Guinness was plunged into the sometimes squalid world that revolved around her parents' unconventional lifestyle. "Everyone was always welcome. So all the liggers came." Not that all the visitors were opportunists: her godmother, Marianne Faithfull, helped Guinness's mother to stop drinking.

Her parents split up when she was 12, and she chose to stay at Leixlip with her grandparents. But she seems remarkably unresentful: "My parents went away to grow up and live their own lives and do all the things they missed out on because of me. It was very unselfish of them as they knew I didn't need to be moved about to strange places. Only now that I am a parent myself, do I realise how heartbreaking it must've been for them to leave me."

"It's great to know where your roots lie and where everything truly began, because that goes to the heart of your identity. We are here, and that in itself is an achievement of our ancestors."