In 1629 King Charles I created him Viscount FitzWilliam in recognition of his family's long record of service to the Crown.
[3] It appears that a good deal was expected in return for the title, and Thomas had to make substantial gifts of money to the King.
When the Irish Rebellion of 1641 broke out, FitzWilliam, unlike many of the nobility of the Pale, remained loyal to the Crown, despite strong pressure from his mother's family, the Prestons of Gormanston, and particularly the military commander Thomas Preston, 1st Viscount Tara, to join the Irish Confederacy.
[4] He was one of only three nobles to offer their assistance in the Royalist defence of Dublin, and a garrison was placed in Merrion Castle.
He offered his services to the Lords Justices, but despite his long record of loyalty to the Crown he was rebuffed, apparently on account of his known leanings towards the Roman Catholic religion, and he took no further part in the fighting.
In his last years he led an unsettled existence, sometimes living at Merrion Castle, sometimes with his eldest son at Howth, and sometimes in Louth.
This is especially interesting since Ormonde in later years was no friend to Fitzwilliam's son Oliver, who played a careful double game during the Cromwellian era and was accused of divided loyalties.