O’Brien is born near Granard, County Longford, in 1804 or 1805. He goes to a local church school, where one of his teachers recognises his intellectual abilities and arranges for him to be educated at the progressive Lovell Edgeworth School. In 1822 he proceeds to Trinity College, Dublin, where he wins several academic prizes including the Science Gold Medal. After studying law at King’s Inns, he moves to England in 1829 with the intention of becoming a lawyer in London.
In London he joins the Radical Reform Association where he meets Henry Hunt, William Cobbett, Henry Hetherington and other leaders of the struggle for universal suffrage. In 1836 he joins the London Working Men’s Association.
O’Brien begins contributing articles to Henry Hetherington’s Poor Man’s Guardian. He signs these articles with the pseudonym ‘Bronterre’ and he eventually adopts it as his middle name. He works very closely with Hetherington and when he is imprisoned for publishing an unstamped newspaper, O’Brien takes over the editorship of Poor Man’s Guardian. He and Hetherington also collaborate on other unstamped newspapers such as The Destructive and the London Dispatch. In 1837 he begins publishing Bronterre’s National Reformer. In an attempt to avoid paying stamp duty, the journal includes essays rather than ‘news items.’ During this period, he and Hetherington lead the struggle against the stamp duty and are consistent in their arguments that working people need cheap newspapers that contain political information.
O’Brien is influenced by the socialist writer François-Noël Babeuf, who had been executed during the French Revolution. In 1836 he begins publishing translations of Babeuf’s work in the Poor Man’s Guardian. He also includes Filippo Buonarroti‘s account of Babeuf and the Conspiracy of Equals. He becomes fascinated with the history of radicalism and begins work on books on Maximilien Robespierre, the French Revolution and the English Commonwealth. However, the authorities raid his house in 1838 and seize his manuscripts and the projects are never completed.
In 1838 O’Brien adds his support for a more militant approach to winning the vote that is being advocated by Feargus O’Connor and George Julian Harney through the London Democratic Association. However, he, unlike O’Connor, refuses to support the use of violence to achieve universal suffrage. He argues that the Chartists should adopt a policy that is midway between the petitioning supported by William Lovett and the Moral Force Chartists, and the violence being threatened by O’Connor’s Physical Force group.
After Bronterre’s National Reformer ceases publication, O’Brien works for O’Connor’s Northern Star. His articles play an important role in increasing the circulation of what had become the most important of the radical newspapers. As well as writing for the Northern Star, he also finds time to publish his own newspaper, The Operative.
O’Brien continues to be active in the Chartist movement and in 1840 he is arrested and charged with making a seditious speech in Manchester. He is convicted of sedition and sentenced to eighteen months in Lancaster Prison. When he is released from prison he finds it difficult to continue working with Feargus O’Connor. The two men disagree over the issue of physical force. Another source of dispute concerns parliamentary elections. O’Brien favours the idea of putting up Chartist candidates whereas O’Connor prefers the tactic of putting pressure on the Whig government by threatening to vote for Tory candidates. He is involved in standing Chartist candidates against Government Ministers in key seats, particularly in standing against Henry John Temple, 3rd Viscount Palmerston in Tiverton.
O’Brien finally breaks with O’Connor when, along with Henry Vincent and Robert George Gammage, he joins the Complete Suffrage Union. He continues to publish newspapers. He joins with his old friend Henry Hetherington to revive the Poor Man’s Guardian in 1843 and this is followed by the National Reformer in 1844. These newspapers are not a financial success and by May 1847, both papers cease publication.
After the failure of these two newspapers, O’Brien concentrates on writing for other publications such as Reynold’s Weekly Newspaper and the Glasgow Sentinel. He also gives public lectures and in 1851 he opens the Eclectic Institute in Denmark Street, Soho, London, where adult education classes are offered in English, French, science and mathematics.
By the 1850s O’Brien’s poverty begins to damage his health. He suffers from bronchitis and his Chartist friends attempt to raise money in recognition of the great sacrifices that he had made in the struggle to win universal suffrage and the freedom of the press. However, the damage to his health is so bad that he spends his last years bed-ridden. He dies on December 23, 1864, and is buried in Abney Park Cemetery.