In her new book, Marianne Elliott draws on her experiences of growing up a Catholic in Belfast to trace the forces that have fused the country's politics with religion. Anne McHardy reports

For Marianne Elliott, writing The Catholics of Ulster meant putting aside her rules as a professor of modern history and agonisingly unpeeling her experience of growing up a Catholic in Belfast. "I had to face my ghosts," she says, in a voice that retains Belfast inflections. "Quite frankly I haven't enjoyed it - even issues such as how to describe myself."

Elliott's job in England and her anonymously non-sectarian married name had for years allowed her to avoid the Northern Irish insistence on declaring religious and, therefore, political allegiances. But when she came to write her new book, she found it useful to relate her historic findings to her emotional understanding.

In some ways, intellectually, this slight, elegant woman, who makes breakfast switching between Radio Ulster and RTE to keep abreast of Irish current affairs, has never left Northern Ireland. But 25 years in England have allowed her to distance herself from her working-class nationalist upbringing. It has also allowed Elliott, mother of a school-age son, to avoid confronting difficult issues, such as segregated education.

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The Catholics of Ulster examines the forces that have shaped Northern Irish Catholics and why religion and nationalist politics have become so intertwined. "I do not subscribe to the belief voiced by many public figures associated with Northern Ireland that we must leave the past behind," she writes in the prologue. "The mistrust, prejudices and fears that lay behind the recent TroublesI have existed for a very long time and are unlikely to disappear overnight."

She still considers herself an Ulster Catholic, "for that was the tradition into which I was bornI I have always felt different from Catholics elsewhere in Ireland and this is a common feeling among Ulster Catholics." Yet she has found Ulster Catholics consistently neglected in the histories of Ireland and of Irish Catholicism, as if Irishness and Catholicism so dominated their sense of themselves that they could be safely included with all other Irish Catholics. "If that had been so, we would not have been left with that overwhelming sense of insecurity that is such a feature of the Ulster personality, or with the daily agonising over culture and identity," she writes. For her, this insecurity fed the Troubles.

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Marianne Burns was born into working-class North Belfast, one of four children. Her father was from Northern Ireland, her mother from Kerry, in the South. Her father worked as a commercial traveller for a Catholic firm, but was also an actor, with irregular work in the theatre. Her childhood home on the White City estate - then new and mixed, now bitterly Loyalist - was always full of actors.

It was because of her father, who had a strong interest in history, that she developed her interest in Ireland's rebels, going back to the United Irishmen, the subject of her first book, and Wolfe Tone, the subject of her second. The family hero was Gerry Fitt, the trade unionist and nationalist from the east of Northern Ireland, who was a founder member of the Social Democratic and Labour Party. Lord Fitt stayed in North Belfast after he became a Westminster MP. "We identified with him," Elliott says.

When Elliott's biography of the Protestant hero of Irish Republicanism, Wolfe Tone, was published in 1991, it received considerable acclaim - although Republicans tagged her "revisionist" - and helped her secure the commission for The Catholics of Ulster. The new book begins 8,000 years before Christ. Up to partition in 1921 it looks at the nine counties of historic Ulster; after partition, at the six Ulster counties that became Northern Ireland. It will inevitably attract criticism for saying that rigid segregation between Protestant and Catholics is comparatively recent and that issues surrounding Protestant settlement and the potato famine are more complex than the popular view. It also identifies differences within the Catholic community, not least between communities east and west of the River Bann.

The book took Elliott nine years to write. She started the research when her son was two and when she had just been offered a lectureship at Birkbeck College, London. She took the job, commuting to the capital weekly from her home in Cheshire, leaving her husband, professor of earth sciences at Liverpool University, as midweek sole parent. Two years later, she took up her professorship of modern history at Liverpool, followed quickly by the directorship of the university's Institute of Irish Studies.

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Writing was crammed into moments grabbed from the quality-assurance-driven regime of modern universities and from other, less prosaic, distractions. In 1992, these included serving on the Opshal Commission, which, over 13 months, undertook the mammoth task of asking the ordinary Northern Irish people what future they wanted. It played havoc with her schedules but suited her research. It took her inside the Troubles in a way that she had never been before. It also gave her a taste of the controversy Northern Ireland's politics attracts. Five years before it became accepted political wisdom, the commission recommended that talks should include the Provisional Sinn Fein. "That was the only thing the media noticed," says Elliott, who found herself under a hostile press spotlight.

She found the book difficult to write for other reasons too. The process was painful, she writes, "not because the experience of Ulster's Catholics has been the doom-laden one of Nationalist traditions - though there was enough of that to dispel any idea of this image as pure mythology - but because I have discovered in myself lingering prejudices and sensitivities that I either believed I had left far behind or never recognised in the first place. And I know I am not alone."

Her remaining puzzle is how she would vote were she living in Northern Ireland. The South Antrim byelection last month highlighted the problem: a Unionist was bound to win; there was no clear pro-Belfast agreement peace-settlement candidate; pro-agreement Unionism is in political trouble; and the SDLP and Sinn Fein were fighting for both nationalist votes and profile.

She would have difficulties, she says, before concluding: "A strong pro-agreement candidate would get my vote."

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The Catholics of Ulster is published next week by Allen Lane The Penguin Press, ?20.00.