William O'Brien, 2nd Marquess of Thomond

William O'Brien, 2nd Marquess of Thomond, 6th Earl of Inchiquin, 1st Baron Tadcaster KP PC (I) (1765 – 21 August 1846) was an Irish peer and Chief of Clan O'Brien.

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.

[2] O'Brien was born in Ennistymon, County Clare, to Captain Edward Dominic O'Brien, High Sheriff of Clare and Mary Carrick, daughter of Christopher Carrick and Áine McNally.

His father was the grandson of William O'Brien, 3rd Earl of Inchiquin.

On his death in 1846 his title passed by the same special remainder to his brother James O'Brien, 3rd Marquess of Thomond.