William O'Brien, 3rd Earl of Inchiquin, 8th Baron Inchiquin

William McWilliam O'Brien, 3rd Earl of Inchiquin PC (1662 – 24 December 1719) was an Irish nobleman.

William O'Brien was the son of William O'Brien, 2nd Earl of Inchiquin and Lady Margareth Boyle. He married his cousin Mary Villiers, daughter of Sir Edward Villiers of Richmond and Frances Howard, the youngest daughter of Theophilus Howard, 2nd Earl of Suffolk and Elizabeth Hume.

Like his father, he was attainted in his absence by the Irish Parliament of King James II in 1689. He was Governor of Kingsale in 1693. He signed the declaration and association in defence of King William III after the assassination attempt against the King in 1697.

He was Privy Councillor to Queen Anne and King George I, Colonel of Foot in 1703 and Mayor of Kilkenny from 1704 to 1705. He was made Governor of County Clare.

Children of William O'Brien, 3rd Earl of Inchiquin and Mary Villiers:

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