An English academic is seeking to restore some of the damage done to Kitty O'Shea's reputation, she tells Arminta Wallace

A wronged husband. A passionate affair. A messy divorce. Sex, lies and political stakes of the highest order. A plot for a chicklit novel, perhaps?

No: it's the real-life story of Charles Stewart Parnell and the woman known to history as Kitty O'Shea.

She, you will remember, was married to Parnell's colleague Captain Willie O'Shea. When the relationship was exposed, Parnell refused to resign and the Home Rule Party split. Parnell finally married Kitty O'Shea in June 1891, only to die four months later at the age of 45.

At the Parnell Summer School in Co Wicklow today, the author of a new biography of O'Shea will explain why her role as political emissary between Parnell and the British prime minister William Gladstone makes her a significant figure in modern Irish history. But Jane Jordan, a lecturer in 19th-century literature, admits that it was that divorce which got her interested in O'Shea in the first place.

"I was interested in the fact that Katherine names her sister in the divorce proceedings," Jordan says. "She said O'Shea committed adultery with her sister. Previous biographers didn't seem to understand why she did that; everybody says she did it out of spite. But under British law, that was the only way a wife could get a divorce. Simple adultery wasn't enough - you had to say 'adultery and incest', or 'adultery and buggery'. It had to be suitably horrifying."

"Very few of her letters have survived, unfortunately,"Jordan says. "But when you do get hold of them, she sounds incredibly modern. She tells her husband; 'Look, we've already agreed on a separation. I don't want you to come back and live here, and harass me all day long, telling me I don't bring up the children well enough. And I don't want your mother telling me this and that'. I've read a lot of Victorian memoirs, but hers is a truly contemporary woman's voice."

Jordan insists that O'Shea played a major role on the Irish political stage in the 1880s. "She was English upper-class; her family were all Liberals; her uncle was a chancellor under Gladstone. Parnell was a rebel who kept making speeches in parliament and getting arrested and thrown out. She was the one who brought Parnell's ideas about local government for Ireland to Gladstone - without her, I don't know how on earth the two men would have communicated." Unlike most historians of the period, Jordan is convinced that Gladstone knew exactly who he was dealing with when he accepted O'Shea as Parnell's emissary.

The official line is, Gladstone didn't believe in gossip; he was a pure-minded Christian who wouldn't believe that Parnell could have an affair with a married woman.

"But in her first letter to Gladstone, O'Shea notes that she hasn't told her husband about the talks. Now, what Victorian wife would write to the prime minister saying, 'I am the confidante of Mr Parnell - but let's not tell my husband. . .'?"

The woman known to history as Kitty O'Shea was christened Katharine. Her friends called her Katie. "Kitty", a slang term for a prostitute in the late 19th century, was a calculated insult hurled at her by outraged Parnellites. Yet Jordan has subtitled her biography "the dramatic life of Kitty O'Shea". "That's how we all know her," she says. The lady, it seems, is not for turning - yet.

• Jane Jordan will address the Parnell Summer School at Avondale, Rathdrum, Co Wicklow, today. Her book Scandal, Secrecy and a Tragic Love Story: the Dramatic Life of Kitty O'Shea, is published by Sutton in September