William O'Brien, 4th Earl of Inchiquin, KB, PC(I) (1700 – 18 July 1777) was an Irish peer, Chief of Clan O'Brien, and Whig politician who sat in the House of Commons between 1722 and 1754.
On 29 March 1720, he married his cousin, Lady Anne Hamilton, the eldest daughter and heiress of George Hamilton, 1st Earl of Orkney, and his wife Elizabeth (née Villiers). His wife became 2nd Countess of Orkney in her own right in 1737. They had five children:
Lady Mary (c.1721–1791), later 3rd Countess of Orkney
William, styled Lord O'Brien (1725–1727)
George, styled Lord O'Brien (1727–1728)
Augustus, styled Lord O'Brien (died in infancy)
Murrough, styled Lord O'Brien, later styled Viscount Kirkwall (1731–1741)
Additionally, Inchiquin was feted by antiquarians and many of the works of Irish history produced at this time are dedicated to him; as an Anglican deriving his lineage from an old Gaelic family, he was a politique choice for those wishing to disseminate their work in Ascendancy Ireland and Hanoverian Britain.