He was created Earl of Kerry on 17 January 1723, together with the subsidiary title Viscount Clanmaurice, both in the Peerage of Ireland. He was notorious for his hot temper, and even in an age when dueling was commonplace, he was reprimanded for challenging John Methuen, the Lord Chancellor of Ireland to a duel. They had five children:
William Fitzmaurice, 2nd Earl of Kerry
John Petty, 1st Earl of Shelburne
Elizabeth, who married Maurice Crosbie, 1st Baron Brandon
Arabella, who married Colonel Arthur Denny
Charlotte, who married Sir John Colturst, first of the Colthurst baronets of Ardrum.
Thomas FitzMaurice was the grandfather of William Petty, 2nd Earl of Shelburne, Prime Minister of Great Britain. the most severe and inflexible character that can be imagined, obstinate and inflexible.. his family did not love him but dreaded him, as did his servants".
FitzMaurice was a Member of the Irish House of Commons for Kerry from 1692 to 1697.