Fitzsimon was born at Broughall Castle, the son of a Catholic military officer, landowner and justice of the peace. His mother was a member of the Magawly family, counts of the Holy Roman Empire.
In 1829 Fitzsimon had married the daughter of the eminent Dublin whiskey distiller, Sir John Power, an alderman of the corporation and ‘one of the most efficient supporters’ of Daniel O’Connell.
Fitzsimon opposed the removal of the malt tax, 27 Feb. 1834 (and would do so again, 10 Mar. 1835), and divided against Lord Althorp’s motion for the replacement of church rates with a central grant raised from land tax, 21 Apr. He did, however, vote for the repeal of the Union, 29 Apr. 1834 (O’Connell had to contradict a report that Fitzsimon had behaved towards him ‘in an unfriendly manner’ during Henry Lambert’s attack upon the Irish leader in the debate).
In 1835 Fitzsimon was re-elected unopposed as a Reformer, presenting himself as ‘the undeviating supporter of the people’s rights’, an ‘uncompromising opponent’ of Irish tithes, and an enemy of the ‘Tory administration’.
By 1837 Fitzsimon was generally regarded as a Whig and was returned unopposed at the general election.
Fitzsimon’s loyalty to the Whig ministry was eventually rewarded, and he unexpectedly took the Chiltern Hundreds, 5 Feb. 1841, in order to assume the ‘comparatively insignificant’ office of a Dublin police magistrate.
Fitzsimon remained in government service until his sudden death in July 1849, when he was seized with ‘an attack of apoplexy’ at his seat, Broughall Castle. A monument was subsequently erected to his memory in Kilcormac Church.
