He was born at Dromoland in Newmarket-on-Fergus, County Clare, on 17 October 1803, the second son of Sir Edward O'Brien, 4th Baronet of Dromoland Castle. His mother was Charlotte Smith, whose family owned Cahermoyle in County Limerick. When William inherited that property, he added Smith to his surname out of respect to his mother's line, and lived at Cahermoyle House a mile from Ardagh in west Limerick. The O'Briens of Dromoland traced their descent from Brian Boru, the High King of Ireland who fell at Clontarf in 1014, through nine centuries of intermarriage with English settlers and Gaelic chieftains. William received the education his class assumed: Harrow School, Trinity College Cambridge, then law at King's Inns Dublin and Lincoln's Inn London. He entered Westminster in 1828 as MP for Ennis, transferred to County Limerick in 1835. A Protestant who supported Catholic Emancipation, he was - until the Famine - a supporter of the British-Irish union.

The break came slowly. In 1843 he joined Daniel O'Connell's Repeal Association in protest at O'Connell's imprisonment, but O'Connell's commitment to constitutional methods and his rejection of any resort to force frustrated the younger members. When O'Connell and his son John forced a vote in 1846 that bound members to non-violence regardless of circumstances, Smith O'Brien led a walkout with the Young Irelanders. They formed the Irish Confederation. Smith O'Brien continued to preach reconciliation with O'Connell until the older man's death in May 1847, but by then the Famine had begun in earnest. The Confederation's leader Charles Gavan Duffy elevated Smith O'Brien - a Protestant, a landowner, a man whose name carried weight - to the public leadership. Smith O'Brien began organizing practical famine relief. By the spring of 1848 the catastrophe had convinced the entire Confederation Council that independence was an existential question. In March he called for the formation of a National Guard. He was arrested for sedition. He was acquitted.

The British government chose coercion. John Mitchel was convicted under new martial law measures, transported. On 9 July 1848 Gavan Duffy was arrested. The issue of the Nation that would have carried his call to arms was seized; the paper was suppressed. Planning fell to Smith O'Brien. He had hoped for French assistance - Ledru-Rollin in Paris had loudly proclaimed French support - and for an Irish-American brigade, and for a Chartist diversion in England. None of it materialized. On 23 July he raised the tricolour in Kilkenny and moved through Tipperary with John Dillon and Meagher. Curious crowds came out to look at him, but only a few hundred ill-armed men joined. The decisive moment came at Ballingarry. A constabulary patrol took refuge in a slate-roofed farmhouse belonging to the Widow McCormack, taking her five children hostage. Smith O'Brien tried to negotiate. Shots were exchanged, two of his men were killed, the column scattered. The Times of London immortalized it as 'the Battle of Widow McCormack's Cabbage Patch.' He was tried for high treason and sentenced to be hanged, drawn, and quartered - the last Irishman to receive that medieval sentence. Petitions for clemency carried 70,000 names in Ireland, 10,000 in England.

On 5 June 1849 the sentence was commuted to transportation for life to Van Diemen's Land - Tasmania, on the far side of the world. Smith O'Brien was held first on Maria Island, where he tried to escape by hiring a schooner whose captain, Ellis, betrayed him; he was then transferred to the harsher Port Arthur penal settlement, where he met John Mitchel, transported before him. The cottages he lived in on Maria Island and at Port Arthur are now preserved as memorials. In a small act of poetic justice, when Ellis later emigrated to the United States he was tried in San Francisco by Terence MacManus - another exiled Young Irelander - in a lynch court for his betrayal. He was freed for lack of evidence. Smith O'Brien received a conditional pardon in 1854 and an unconditional one in May 1856. He returned to Ireland in July of that year. He published Principles of Government, or Meditations in Exile, contributed to the Nation, and refused George Henry Moore's plea that he lead the Independent Irish Party. He travelled to Wales in 1864 hoping the change of air might rally his health. He died at Bangor on 16 June, two days short of his 61st birthday. His sister Harriet had become Mother Harriet Monsell, founder of an Anglican order of nuns at Clewer near Windsor; she wore until her own death a gold cross made from gold William had panned in Australia. A statue in Portland limestone, sculpted by Thomas Farrell, stands today in O'Connell Street, Dublin.