Thomas's father spent some years in France and fought for Charles I in England during the English Civil War, becoming Governor of Whitchurch.
[2] In 1655 William and his elder brother Oliver FitzWilliam, 1st Earl of Tyrconnell, having made their peace with the English Parliament, were allowed to recover part of the family estate.
[3] Thomas shared his father's religious beliefs: according to tradition he gave money for the building of a Catholic Mass house at Booterstown.
When James II succeeded to the throne Thomas enthusiastically supported his pro-Catholic policy and for the next few years he was a political figure of some importance.
Lord FitzWilliam married firstly Mary Stapleton, daughter of the leading Parliamentarian Sir Philip Stapleton and his first wife Frances Hotham: this was a somewhat surprising marriage, given the deep divisions about religion and politics between the two families.