Thomas FitzMaurice, younger son of John Petty, 1st Earl of Shelburne.
His mother was Mary, suo jure Countess of Orkney, daughter of Murrough O'Brien, 1st Marquess of Thomond and Mary, suo jure Countess of Orkney.
The Prime Minister of Great Britain, Lord Shelburne, William Petty FitzMaurice, was his uncle.
He gained the courtesy title Viscount Kirkwall when his mother succeeded as Countess of Orkney in 1791.
Kirkwall was returned to Parliament for Heytesbury in 1802, a seat he held until 1806, and later represented Denbigh Boroughs between 1812 and 1818.