William Plunket, 1st Baron Plunket

William Conyngham Plunket, 1st Baron Plunket (1 July 1764 – 5 January 1854) was an Irish politician and lawyer who eventually became Lord Chancellor of Ireland.

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After the Act of Union was passed, Plunket lost his seat, but he subsequently became Solicitor General for Ireland in 1803, a post he held for two years before becoming the country's Attorney General, again for two years. He was appointed a member of the Privy Council of Ireland on 6 December 1805.

In January 1807, he was returned to British House of Commons as a Whig member for Midhurst, representing the constituency for only three months, although he subsequently returned to the House of Commons in 1812 as a member for Dublin University. Between 1822 and 1827, he was again Attorney General for Ireland, and in the latter year he became the island's Chief Justice of the Common Pleas.

In 1827, Plunket was ennobled in the Peerage of the United Kingdom as Baron Plunket, of Newton in County Cork. He was an advocate of Catholic Emancipation, and served as Lord Chancellor of Ireland from 1830 to 1841, with a brief interval when the Tories were in power between 1834 and 1835. He was forced into retirement to allow Sir John Campbell to assume office, and died at the age of 89 at his home in County Wicklow.

The title was inherited by his eldest son, Thomas, became Church of Ireland Bishop of Tuam, Killala and Achonry. Thomas's eldest daughter Katherine (1820 – 1932) was the longest-lived Irish person ever.