Admiral of Leinster merchant of Dutch
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.
They had several children:
Richard FitzWilliam, 7th Viscount FitzWilliam, (c 22 August 1745, Richmond - 1816)
Honorary Henrietta FitzWilliam, (c 16 October 1746 Richmond - )
Honorary
William Fitzwilliam, (c 18 September 1749 Richmond - )
Honorary Mary FitzWilliam, (c 20 August 1748, Richmond - )
John FitzWilliam, 8th Viscount Fitzwilliam, (c21 October 1752 Richmond – 1830)
Honorary
Catherine FitzWilliam, (c30 October 1753 Richmond - )
Thomas FitzWilliam, 9th Viscount Fitzwilliam (c3 September 1755 Richmond - January 1833).
Royal Society.
