William FitzWilliam, 3rd Viscount FitzWilliam Irish noble (d. 1670) Parents Margaret Plunkett Thomas FitzWilliam, 1st Viscount FitzWilliam Children Thomas FitzWilliam, 4th Viscount FitzWilliam Catherine Fitzwilliam Siblings Oliver FitzWilliam, 1st Earl of Tyrconnell Richard Fitzwilliam