Is one of the most reviled figures in Irish history due rehabilitation? A controversial new book traces the founding of Irish Republicanism directly back to Oliver Cromwell. In Dissent Into Treason, writer and trade union official Fergus Whelan says those who came to Ireland as part of Cromwell's army not only made a significant contribution to radical politics, but were the founders of the Society of the United Irishmen in 1791.

While history already records that this secret society united Belfast Presbyterians with southern Catholics, it's been the view that this was a temporary arrangement. Not so, says Whelan, whose research is based on hidden records from the Meeting House in Great Strand Street which, during the late 18th century, housed the Dublin Unitarian Church.

"These Protestant dissenters who came to Dublin as part of the Cromwellian settlement were always radical, and remained radical, with freedom of conscience as their core belief," says Whelan. "They had basic republican and democratic structures in their religious observance, but also advocated those structures in civil society as well. Their belief in religious liberty is the very essence of what the United Irishmen were about – and that came from the Cromwellian tradition of English Protestant dissent."

The leader of 'The King Killers of Pill Lane', a group of Protestant radicals within the United Irishmen, was Oliver 'Cromwell' Bond, and the book claims the regicide referred to here was not, as usually perceived, that of Louis XVI and the French revolution that inspired the rebels of 1798, but the execution of Charles I in 1649 by Cromwell's followers. The established Anglican church was also considered the enemy of religious tolerance, with bishops, as well as kings, regarded "tyrants" by the free-thinking Dissenters within the United Irishmen.

This period of the late 18th century has been under-explored, said Diarmaid Ferriter, Professor of Modern Irish History at UCD, who has read the manuscript of the book and agrees that many will be surprised by its revelations. "Sometimes we have an overly simplistic version of the origins and evolutions of those events. When the various layers are stripped back, that will always complicate the simple narrative that we may have in our minds. The more research is carried out, the more you have to call into question what has been traditionally accepted."

A particular grievance challenged by Whelan in his research is the massacre of Catholics carried out in Drogheda by Cromwell in 1649 when the garrison refused to surrender. "Drogheda was a Protestant town, and I believe Cromwell was lying, and knew he was lying, when claiming the people slaughtered were Catholics," says Whelan. "His reason for that was to make the massacre more acceptable back in England."

Should Ireland take a less caustic view of 'Old Ironsides' now? Not quite, says Whelan. "While I have come to have a huge regard for the radical Protestant ministers that Cromwell planted in Dublin, I don't have a great regard for the man himself. In fairness to him, he also believed in freedom of conscience and religious tolerance among the soldiers in his New Model Army. But he was resolutely anti-Irish."

Whichever way the findings are received, the writing of them proves one unarguable fact among historians, said Ferriter. "This kind of book is a reminder of the importance of the preservation of records, such as these old church documents in the Unitarian church which have been carefully maintained over the centuries."