William O'Brien, 2nd Marquess of Thomond, 6th Earl of Inchiquin (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.
He was a founding member of The Yacht Club.