William Smith O'Brien (Irish: Liam Mac Gabhann Ó Briain; 17 October 1803 – 18 June 1864) was an Irish republican who, in the course of Ireland's Great Famine, had been converted to the cause of national independence while sitting as a unionist member of the United Kingdom Parliament. Returning from revolutionary Paris (the Second French Republic) with the first Irish tricolour, in 1848 he attempted an armed rebellion. With fellow "Young Irelanders" he was convicted of sedition and transported to Van Diemen's Land (Tasmania). Pardoned, in 1856 he returned to Ireland, where he published an unrepentant memoir and, through Ossianic Society, promoted the study and revival of the Irish language.

Born in Dromoland, Newmarket on Fergus, County Clare, William Smith O'Brien was the second son of Sir Edward O'Brien, 4th Baronet, of Dromoland Castle. His mother was Charlotte Smith, whose father owned a property called Cahirmoyle in County Limerick. William took the additional surname Smith, his mother's maiden name, upon inheriting the property. He lived at Cahermoyle House, a mile from Ardagh, County Limerick. He was a descendant of the eleventh century Ard Rí (High King of Ireland), Brian Boru. He received an upper-class English education at Harrow School and Trinity College, Cambridge. Subsequently, he studied law at King's Inns in Dublin and Lincoln's Inn in London.

From April 1828 to 1831 Smith O'Brien was the Tory faction MP for Ennis, his father's borough. Although a Protestant country-gentleman, he supported Catholic Emancipation and the Roman Catholic Relief Act 1829 while remaining a supporter of British-Irish union.

In 1835 Smith O'Brien became Whig MP for County Limerick. In 1837 Daniel O'Connell clashed with him over his opposition to the introduction of secret voting in elections and also Smith O'Brien's support for granting state payments to Catholic clergy. The Catholic Bishops came out in support of O'Connell's stance on Church and nation, resolving "most energetically to oppose any such arrangement, and that they look upon those that labour to effect it as the worst enemies of the Catholic religion."

Smith O'Brien remained in the House of Commons until 1849 when his seat was forfeited.

In 1843, in protest against the imprisonment of Daniel O'Connell, he joined O'Connell's anti-union Repeal Association. Within the association he identified with the circle around Charles Gavan Duffy and his paper The Nation which O'Connell in hostile reference to Giuseppe Mazzini's anti-clerical and insurrectionary Young Italy dubbed Young Ireland.

After O'Connell and his son John forced a division with resolutions renouncing a resort to revolutionary force regardless of circumstances, Smith O'Brien withdrew with the Young Irelanders into a new Irish Confederation, although he was to continue to preach reconciliation until O'Connell's death in May 1847. The objectives of the Confederation were "independence of the Irish nation" with "no means to attain that end abjured, save such as were inconsistent with honour, morality and reason".

In the Confederation Duffy was trying to hold together a broad national coalition, and had for that reason advanced Smith O'Brien, as a Protestant and a landowner, to the leadership. On the Confederation's Council Duffy and Smith O'Brien were supported by Patrick James Smyth who argued that with propertied classes, as well as the priesthood opposed, the Confederation could not, in the event of insurrection, hope to call out a single parish in Ireland.