William Parnell-Hayes (c.1780 â 2 January 1821) was an Irish politician. He was member of parliament for Wicklow from 1817 until his death.
Third son of Sir John Parnell, 2nd Baronet, and Letitia Charlotte, daughter of Sir Arthur Brooke, 1st baronet, William Parnell was educated at Eton and Trinity College, Cambridge.
In 1810, Parnell-Hayes married Frances, daughter of the politician Hon. Hugh Howard. He adopted the additional surname of 'Hayes' in 1801 on succeeding his father to the Avondale estate, which had previously been the property of politician Samuel Hayes.