O'Sullivan, William Henry (1829–87), agrarian radical and nationalist, was born in Kilmallock, Co. Limerick, the only son of Thomas Luke O'Sullivan, a prosperous tenant farmer, of Rathkeale, Co. Limerick, and his wife, Nanette, daughter of Patrick Hussey, of Kilmallock. Educated locally, at the age of 18 he married Eliza, daughter of William Spread, owner of Ballynolan House, Co. Limerick. He soon emerged as a successful hotelier and publican in Kilmallock, and became politically active during the late 1860s after the imprisonment of his sole child, William, who had joined the IRB. A strong supporter of the causes of tenant right and amnesty, he denounced Gladstone's 1870 land act as a useless measure and played a leading role in establishing the Limerick, Clare, and Tipperary Farmers’ Clubs. As vice-chairman of the Kilmallock board of guardians, he achieved notoriety by setting a pioneering example of using meetings of poor law boards (then the only elective local government bodies in the countryside) as a platform for championing radical viewpoints on national questions and getting his speeches reported in the Dublin press.
Representing himself as a people's champion, he accused all Irish MPs of the day of being completely unrepresentative, and became a popular figure in Co. Limerick. By then he was well known nationally and was one of the principal speakers at the inaugural convention of the Home Rule League (November 1873). With the support of the Amnesty Association and the farmers’ clubs, he was elected MP for Limerick county at the 1874 general election and was re-elected in 1880 despite facing strong clerical opposition. Together with Joseph Biggar (qv), C. S. Parnell (qv), and John O'Connor Power (qv), he pioneered the use of obstructionist tactics in parliament. In August 1876, after protesting at length in Westminster against the near monopoly held in Ireland by Scottish whisky distilleries, he was nicknamed derisively by critics as ‘Whiskey O'Sullivan’. He was, however, a champion of numerous radical causes, in and out of parliament. These included the right of women to vote and sit in parliament, the complete separation of church and state, and the protection of the interests of small Irish manufacturers, as well as the urban and rural working class.
In March 1878, together with Parnell, he attended a secret meeting convened in London by William Carroll (qv) to discuss the possibility of an alliance between the parliamentary obstructionists at Westminster and the Irish nationalists of the IRB. On the foundation of the Land League (21 October 1879) he was appointed a member of its executive and one of its three treasurers. During 1879–80 he chaired four meetings of the league's central branch and spoke at some of its largest public rallies, though his attempts to give the interests of agricultural labourers priority over those of tenant farmers failed to make any impact. He withdrew from the agitation following the rise of agrarian outrages in late 1880, while his support for the land act of April 1881, and his refusal to be bound by Irish party discipline, soon caused him to be marginalised by Parnell.
In early 1882, together with J. F. X. O'Brien (qv), he established a wine and tea wholesale business in London but chose to continue living in Kilmallock, as he had done all his life. He remained largely independent of the Irish National League (established October 1882), which, at the clergy's request, persuaded him not to stand for reelection in the 1885 general election. Thereafter he retired from politics and, after a short illness, died 27 April 1887 in his Kilmallock home.