Fitzwilliam, a Roman Catholic, conformed to the established church, taking his seat in the Irish House of Lords on 25 May 1710. In 1715 he was a member of a committee of Irish Lords to congratulate George I on his accession.
Lord Fitzwilliam’s performance was darker, thicker and heavier than the fog of this day ... I am sorry when God thought fit to send such a piece of original obscurity and chaos into the world as that head, that he did not think fit to dispel the mists of it by the same methods which Moses tells us he made use of to enlighten the rest of the universe. Human aids, I am sure, can never bring it about.
Ilchester, Lord Hervey and his Friends, 44.
Never standing again, he died 6 June 1743.