The prerequisites for becoming a best-selling Irish writer are an abusive, impoverished childhood and an oppressive Catholic upbringing. Add the stigma of illegitimacy, religious persecution, four miscarriages, a split personality, paranoia, a belief in the paranormal, an intense emotional lesbian attachment, blackmail, nervous breakdowns and the chronic bleeding of internal membranes caused by haemmorrhagic Telangietasia, and you have the writer you would probably least suspect as the classic Irish psychological disaster area: Dame Catherine Cookson.

Cookson has been condemned by intellectuals as a purveyor of mass-market mush. But her fans love her because her novels are about personalities, not ideas. When Leeds University attempted to run a course on her work, only one person signed up. In the US, Catherine's work is regarded as social history - and it does shed light on the Irish Catholic culture in which child abuse and incest were the norm outside, as well as inside, industrial schools and orphanages.

If you are prone to finding yourself sitting tucked under the comforter with a box of chocolates and a Cookson, you may already know that there is nothing comforting about the Dame, a great survivor who died last year, leaving an estate worth £20 million sterling. (Her 97 novels, all of which are still in print, have sold 128 million copies to date.) Every one of her heroines is drawn from her own appalling childhood, a living hell that makes Angela's Ashes look like a jolly fairy tale. But the most disturbing theme in her life was her relationship with a lesbian, Nan, who threatened to shoot Cookson on her wedding day, and blackmailed her for nearly 30 years. Like many adults who had disturbed childhoods, Cookson escaped geographically, but not emotionally. "When did I first start to be a victim?" she once asked herself. Her adolescent experience had led her to expect persecution - for being illegitimate, for being a Catholic, for being different from other children and for having violent tendencies. Because of her deep Catholic guilt, she could not defend herself against others' needy behaviour, describing herself as "strings for other people to play on". She was Nan's violin.

Cookson is associated with the "Geordie" accent of her native Newcastle, but her literary cradle was actually Irish story-telling by the fire - and a pitiful fire it was. As a young child, Cookson would finger through other people's dustbins to find lumps of spent coal to bring home. Most of her family were Irish, violent, alcoholic and religiously obsessed. They all lived together in the "Fifteen Streets" (after which she named one of her first novels), an area of poor housing that was the rock bottom of Tyneside, which in turn was the rock bottom of the Western world.

In the 1910s and 1920s when Catherine was growing up, illegitimacy was regarded as an inherited sin, for which the child deserved to be punished. Everyone in the Fifteen Streets condemned Catherine as illegitimate, yet her family pretended that she wasn't. As she grew older, it began to dawn on her that her "mother" was actually her grandmother, and her "sister", Kate, was really her mother. It was a classic dysfunctional set-up, and made Cookson angry and resentful.

Kate, meanwhile, reared her daughter to believe that she was superior to the neighbours and suggested that Catherine's father was a man of higher class. Cookson claimed to know his identity, and brought the secret with her to the grave. This fantasy of superiority was contradicted by the fact that her mother and grandparents sent her to the pawnshop - followed by the pub to fetch drink - on a daily basis. Catherine found this so humiliating that all her adult life, she was terrified of going into shops, and always bought her clothes from catalogues. But the poverty wasn't the worst of it: Cookson was sexually assaulted by her brother (Cookson's biographer, Kathleen Jones, defines an Irish virgin at that time as a girl who could run faster than her brother).

She was also sexually molested by her mother's Irish fiance, and witnessed her grandfather's sexual assaults on her own mother, with whom she shared a bed. And her mother, Kate, an alcoholic, lashed out at her daughter when she was drunk. Cookson survived, as many disturbed children do, by detaching herself from reality. She developed a split personality: one a bright, extrovert with a deceptively open personality; the other a dark, malicious, aggressive "twin". She also had a lifelong fear of rising in her sleep to kill Kate. The pull of the evil twin was so strong that Cookson sometimes felt irresistibly drawn towards evil deeds, even in adulthood. She was once tempted to kidnap and murder a baby she saw sleeping in a pram outside a shop, and only stopped when a workman noticed her confusion. Nan played on Catherine's dark side. After Cookson escaped her miserable home in Tyneside as a young woman, she took a job managing a laundry in a workhouse.

It was here that she befriended Nan, who soon came to live in her apartment. Catherine later bought a 15-bedroom mansion in Hastings, in which they lived and operated as a home for "mental defectives". While, externally, Catherine played out her "lady of the manor" fantasy in her imposing house, internally she behaved with exquisite self-destructiveness - taking in her alcoholic mother, Kate, as well as Nan, and turning Hastings into "a hell of jealousy with women".

Nan regarded herself as Catherine's partner - as did Catherine's friends. At least one other bisexual woman attempted to seduce Catherine, believing that her lifestyle was lesbian. Catherine hid all this from her husband, Tom, who was seven years younger and, as the son of a domineering Protestant mother, was only too willing to be bullied by his wife. Jones suggests that Catherine deliberately set up a situation in which Nan and Tom were fighting over her, although she denied that anything sexual went on. For 26 years, however, Cookson supported Nan by buying her property, but lived in constant fear that she would show Tom their letters.

Thanks to Tom Cookson's devotion, Catherine became a full-time writer at the age of 40, often composing her novels and entertaining in bed. She wrote her story over and over again, and was in her element as the poor girl turned rich and famous writer. Tom, a teacher, paid the price by suppressing his own ambitions to support his wife. After 26 years of marriage, he also suffered the disappointment of discovering Nan's letters, which contained material that nearly wrecked the Cookson marriage.

Tom remained faithful, however, and when his wife died, he felt so empty and confused that he died of a heart attack three weeks later - a victim in his own right.

Catherine Cookson: The Biography by Kathleen Jones (Constable and Company, £16.99 in UK)