Richard FitzWilliam, 6th Viscount FitzWilliam
FitzWilliam was the eldest son of Richard FitzWilliam, 5th Viscount FitzWilliam, by Frances Shelley, daughter of Sir John Shelley, 3rd Baronet. He succeeded his father in the viscountcy in 1743 and took his seat in the Irish House of Lords. He was Vice-Admiral of Leinster and sworn of the Irish Privy Council in 1766. He was a Fellow of the Royal Society.
He lived for a time with his father-in-law, Sir Matthew Decker, a wealthy merchant of Dutch origin, at Richmond Green, but moved back to Mount Merrion, the family's principal seat in Dublin. He was buried at Donnybrook Cemetery.
Lord FitzWilliam married Catherine Decker, eldest daughter of Sir Matthew Decker, 1st Baronet and his wife Henrietta Watkins, on 3 May 1744: Horace Walpole thought the marriage notable enough to mention in his correspondence. They had several children:
- Richard FitzWilliam, 7th Viscount FitzWilliam, (c. 22 August 1745, Richmond - 1816)
- Hon. Henrietta FitzWilliam, (c. 16 October 1746 Richmond - )
- Hon. William Fitzwilliam, (c. 18 September 1749 Richmond - )
- Hon. Mary FitzWilliam, (c. 20 August 1748, Richmond - )
- John FitzWilliam, 8th Viscount Fitzwilliam, (c.21 Oct 1752 Richmond – 1830)
- Hon. Catherine FitzWilliam, (c.30 Oct 1753 Richmond - )
- Thomas FitzWilliam, 9th Viscount Fitzwilliam (c.3 September 1755 Richmond - January 1833) 
- Letters of Horace Walpole, Earl of Orford, to Sir Horace Mann Richard Bentley London 1833 Vol. 2 p.12
- Sir Bernard Burke (1866). A genealogical history of the dormant, abeyant, forfeited, and extinct peerages of the British empire. Harrison.
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