Ireland's Misfortune, by Elisabeth Kehoe

When Lloyd George, then Chancellor of the Exchequer with every chance of succeeding to the premiership, embarked on an affair with Frances Stevenson, he gave her a biography of the Irish leader Charles Stewart Parnell as a dreadful warning. The obvious message was that he was not going to let his career be wrecked, as Parnell's had been by the revelation of his adultery with the wife of a parliamentary colleague, Katharine O'Shea, in 1890. The second moral of Parnell's downfall was that it is always the woman who pays the heavier price.

Parnell died within a year of the divorce case in which he was cited by Katharine's husband as co-respondent, his title as the "uncrowned king of Ireland" badly tarnished. But his reputation quickly recovered; and the longer Home Rule was postponed, the more he was seen to have embodied the best hope of achieving it, until brought down by a scheming harpy. In nationalist demonology "Kitty" O'Shea was vilified as a wicked Jezebel – "that bitch, that English whore", as James Joyce called her – who had set back Irish freedom by a generation.

In fact Katie, as she was always known, was an exceedingly well-connected woman, cultured, intelligent and astute. Her father was a baronet, her uncle Lord Chancellor in Gladstone's first government, and her brother, Sir Evelyn Wood, a much-decorated soldier. Unfortunately, she made a bad marriage. Captain Willie O'Shea was a plausible bounder who lost money at everything he touched before becoming MP for Co. Clare in 1880. Katie met Parnell through trying to advance her husband's career by inviting his leader to dinner. They quickly became lovers. Parnell was unmarried; but he used her to conduct delicate negotiations with Gladstone, who treated her role as a go-between seriously. She was not a wrecker but a constructive intermediary. Moreover, her husband was not deceived, but turned a blind eye to advance his own political prospects.

Ireland's Misfortune is presented as an overdue rehabilitation of a woman long maligned by history. But Elisabeth Kehoe's is the fourth biography in 30 years – the most recent in 2005 – and adds little new. Thoroughly researched and filled with a wealth of social detail, the book is slightly unfocused, as if the author is unsure whether she is writing a romance or a forensic investigation. This is a serious weakness with a story so tangled as the Parnell-O'Shea triangle.

The problem is that so much of the evidence is missing, and much of what we have is questionable and contradictory. The prime source remains Katie's own highly misleading account, published in 1914, in which she printed extracts of Parnell's letters to her but none of hers to him. The first difficulty lies in judging how much can be believed. The second is that it was not a straightforward story of adultery and deception, nor even a great love kept secret for political reasons.

The unromantic truth is that it was all about money. Both the O'Sheas were dependent on Katie's wealthy aunt. Willie could not afford to divorce Katie, his meal ticket; Katie could not afford a scandal which would jeopardise her inheritance.

For nine years Katie skilfully managed to maintain reasonable relations with Willie and a good home for their three children (plus two of Parnell's). Only when "Aunt Ben" died, aged 97, leaving her whole estate to Katie, did O'Shea decide – for a share of the loot – to join with her three siblings to contest the will and sue for divorce as part of that action. Until the last minute, Parnell and Katie hoped to buy him off, which is why they did not contest the suit. Had they proved his connivance they would not have got the divorce which they now wanted in order to marry each other.

As in a Trollope novel, the action all turned on the contested will; unfortunately, the biographer lacks the novelist's omniscience, and much of the motivation remains hard to fathom. The lovers did marry; but Parnell died three months later, so Katie was left a widow but an outcast, with very little money. She died in poverty in 1921. The mistress's lot is not a happy one.

John Campbell's 'If Love Were All: the story of Frances Stevenson and David Lloyd George' is published by Vintage