William FitzWilliam, 3rd Viscount FitzWilliam

William FitzWilliam, 3rd Viscount FitzWilliam (c.1610–1670) was an Irish nobleman of the Stuart era. He fought on the Royalist side during the English Civil War, but later made his peace with the Cromwellian regime. In his later years he openly professed the Roman Catholic faith.

He was born at Merrion Castle, at present-day Mount Merrion, Dublin, the youngest son of Thomas FitzWilliam, 1st Viscount FitzWilliam and Margaret Plunkett, daughter of Oliver Plunkett, 4th Baron Louth. The FitzWilliam family were first recorded in Ireland about 1210, and by the seventeenth century had become among the largest landowners in Dublin. They had a record of loyalty to the English Crown, and the viscountcy was conferred on the elder Thomas in 1629 by Charles I in recognition of his good services. The title did not come cheap since the Crown expected financial help in return, and the estates were mortgaged in the 1630s to cover the cost of the family's gifts to the Crown.

As a young man William appears to have lived very much in the shadow of his elder brother Oliver FitzWilliam, 1st Earl of Tyrconnell. When Oliver became a Colonel in the French Army, William was his lieutenant. Both gained the trust of Charles I of England, and while Oliver returned to fight in Ireland, William became Governor of Whitchurch and Lieutenant General for Shropshire.

When it became clear that the Royalist cause was lost, Oliver and William both made their peace with Oliver Cromwell and in 1655 after the deaths of their father and their eldest brother Richard, a portion of the family lands were restored to them. Before the fighting became serious, William and his wife and family had lived at Dundrum Castle, but they were driven out in 1642, returning again in 1646. In about 1652, the Fitzwilliams left Dundrum and did not live there again. Their later home was Simmonscourt Castle, where they were living shortly after the Restoration of Charles II

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