A new book about Daniel O'Connell, 'the Liberator' of Ireland, has unearthed fresh evidence that portrays him as a womaniser who squandered fortunes and fought duels.

According to historian Patrick M Geoghegan, author of King Dan, O'Connell - the founding father of Irish nationalism - was a rakish, aggressive man who fathered children outside his own marriage.

The Trinity College Dublin academic said that his aim was to show the other side to the man who secured Catholic emancipation and tried to repeal the Act of Union. The young O'Connell was 'lazy, aggressive, reckless, uncertain and unsure', according to Geoghegan, who said he wanted to challenge the conventional historical wisdom that reports of O'Connell being an feckless adulterer were simply British propaganda.

On O'Connell's adultery, Geoghegan cites the case of Ellen Courtenay, a young girl from Cork, who gave birth to a son she later claimed was fathered by O'Connell. She alleged in 1832 that she had been raped by O'Connell, although Geoghegan says there is no evidence to sustain that claim.

'While he did not rape Ellen Courtenay in 1818, as she later accused him in print, he almost certainly had an affair with her and a number of other women. O'Connell was flawed,' writes Geoghegan. 'Stories of O'Connell's philandering have been too readily dismissed by historians, often for quite preposterous reasons. Folklore experts have suggested that these stories were "a product of the folk-mind", in other words that peasants wanted their Gaelic heroes to possess "insatiable sexual" powers and so projected tales of philandering on to O'Connell.

'That these stories were therefore a tribute to his popularity does not convince. Others have insisted that any adultery was impossible given that the "powerful evidence" which exists - the tender love letters between O'Connell and his wife which were exchanged throughout their marriage. But if frequent expressions of love precluded adultery, affairs would be a novelty.

'It should also be remembered that O'Connell and his wife had received letters in 1812 and 1816 alleging various infidelities, and these can hardly be seen as products of the folk-mind or as politically motivated attacks. O'Connell himself told his wife in 1820 that his new religious beliefs meant that he "would not, darling, now be unfaithful to you even by a look". Everything depends on the emphasis that is placed on the "now",' Geoghegan writes.

The author turns to papers from O'Connell's brother James to outline the kind of debts the lawyer-turned-MP ran up. 'James calculated that his brother would have to use all this income just to pay the interest on his debts.'

Geoghegan also highlights an incident in 1815 when O'Connell fought a duel with a man called D'Esterre, who died five days later from injuries caused by O'Connell firing into his thigh. On hearing of D'Esterre's death, O'Connell told his brother to hire a Protestant lawyer, fearing that he would be arrested. But, adds Geoghegan, D'Esterre apparently exonerated O'Connell from any blame two days before his death.

Despite a complex personal life, Geoghegan insists that O'Connell was the founding father of the modern Irish state. On his greatest achievement, Catholic emancipation under the Duke of Wellington's government in 1829, Geogeghan points out that O'Connell 'once complained that he would never receive the credit he deserved... because people would never understand the extent of the problems he faced. But he did what was necessary in the context of the time, and he did it without resorting to physical force.'

• 'King Dan: The Rise of Daniel O'Connell, 1775-1829' is published by Gill and Macmillan, price €24.99 (£19.99)