William O'Brien, 2nd Marquess of Thomond
William O'Brien, 2nd Marquess of Thomond, 6th Earl of Inchiquin KP PC (I) (1765 – 21 August 1846) was an Irish peer. He was born in Ennistymon, County Clare, to Capt. Edward Dominic O'Brien and Mary Carrick. He succeeded by special remainder as Marquess of Thomond in 1808 on the death of his uncle Murrough O'Brien, 1st Marquess of Thomond and was appointed a Privy Councillor and Knight of the Order of St Patrick on 11 November 1809.
He was created Baron Tadcaster in the British Peerage in 1826.
On his death in 1846 his title passed by the same special remainder to his brother James O'Brien, 3rd Marquess of Thomond.
William O'Brien married Elizabeth Rebecca Trotter (1775–1852), daughter of Thomas Trotter of Duleek co Meath on 16 September 1799. They had no son but four daughters:
- Susan Maria married Captain later Rear-Admiral George Frederick Hotham RN and they were parents of Charles 4th Lord Hotham
- Sarah married Major William Stanhope Taylor
- Mary married Richard White, Viscount Berehaven who succeeded as 2nd Earl of Bantry
- Elizabeth married George Stucley Bucke who in 1859 was made Sir George Stucley Stucley, 1st Baronet
- Rayment, Leigh. "Knights of the Order of St Patrick". Retrieved 2008-12-13.
|Peerage of Ireland|
|Marquess of Thomond
|Peerage of the United Kingdom|
|New creation||Baron Tadcaster