William Thomas Le Poer Trench, 3rd Earl of Clancarty, 2nd Marquess of Heusden (21 September 1803 – 26 April 1872), styled Viscount Dunlo between 1805 and 1837, was an Irish peer, as well a nobleman in the Dutch nobility. He was educated at St John's College, Cambridge.^[1]

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Trench was born in Castleton, County Kildare, Ireland the son of Richard Trench, 2nd Earl of Clancarty and Henrietta Margaret Staples. On 8 September 1832, he married Lady Sarah Juliana Butler. They had six children.

• Richard Somerset Le Poer Trench, 4th Earl of Clancarty (13 January 1834 – 29 May 1891) married Lady Adeliza Georgiana Hervey
• Major Hon. Frederick Le Poer Trench (10 February 1835 – 17 December 1913) married (1) Harriet Mary Trench (2) Catherine Simpson
• Colonel William Le Poer Trench (17 June 1837 – 16 September 1920) married Harriet Maria Georgina Martins
• Lady Anne Le Poer Trench (1839 – 12 March 1924) married Frederic Sydney Charles Trench
• Power Henry Le Poer Trench (11 May 1841 – 30 April 1899)
• Lady Sarah Emily Grace Le Poer Trench (6 December 1843 – 2 August 1875) married John Melville Hatchell.