He remains a deeply controversial figure in Irish history, being described variously as an old fashioned anti-Catholic Whig political party hardliner and an early advocate of the Act of Union between Ireland and Great Britain (which finally happened in 1801, shortly before his death).

The Earl may also have been the first person to suggest to King George III that granting Royal Assent to any form of Catholic Emancipation would violate his coronation oath.

He entered the Irish House of Commons in 1778 as Member for Dublin University, and held this seat until 1783, when he was appointed Attorney General.

He finally achieved a seat in the British House of Lords in 1799 when created Baron FitzGibbon, of Sidbury in the County of Devon, in the Peerage of Great Britain.

FitzGibbon later claimed that he had been duped by the way in which the Act was passed (by the new Viceroy Lord Cornwallis promising reforms to Irish Catholics), and was bitterly opposed to any concessions during the short remainder of his life.

According to some, he supported a hardline policy which used torture, murder and massacre to crush the rebellion [citation needed], or that as Lord Chancellor, he had considerable influence on military affairs, and that martial law could not have been imposed without his consent.

Lord Clare's former side was displayed by sparing the lives of the captured United Irish leaders, 'State prisoners', in return for their confession of complicity and provision of information relating to the planning of the rebellion.

In contrast to the leniency shown to the largely upper-class leadership, the full weight of military repression was inflicted upon the common people throughout the years 1797–98 with untold thousands suffering imprisonment, torture, transportation and death.

Lord Clare died at home, 6 Ely Place near St. Stephen's Green, Dublin on 28 January 1802 and was buried in St. Peter's Churchyard.

A hero to Protestant hardliners, but despised by the majority Catholic population, his funeral cortege was the cause of a riot and there is a widespread story that a number of dead cats were thrown at his coffin as it departed Ely Place.

In thus convincing the King (probably between 1793 and 1801), FitzGibbon's policy of repression towards Irish Catholics achieved its finest hour; in doing so, he also defeated all that Grattan and his party had worked to obtain.

By negating all of Grattan's efforts 1787-1789 and those of Pitt in the late 1790s to 1801, FitzGibbon allowed conditions to develop that would benefit sectarian leaders and political philosophies from both religious communities.

But just as Catholic Emancipation was brought about by a Tory Prime Minister in 1829, or substantial voting reforms brought about by Disraeli and the Conservatives, thus winning support from a crucial minority of those originally opposed to either, the support of a prominent hardline Protestant leader for Catholic Emancipation might have made all the difference to Irish history.

Sir Jonah Barrington's estimate of Fitzgibbon:- "His political conduct has been accounted uniform, but in detail, it will be found to have been miserably inconsistent.