The American War of Independence awakened much sympathy in Ulster, especially among the Presbyterians, who, being disqualified from holding office, desired a general emancipation including that of the Roman Catholics.

In 1778 the Irish parliament, under the influence of the reformist leader Henry Grattan, passed the Relief Act, removing some of the most oppressive disabilities. Meanwhile Irish Protestants, under the pretext of defending the country from the French, who had entered into an alliance with the Americans, had formed military associations of volunteers, with 80,000 members. Backed by this force they demanded legislative independence for Ireland, and as a result of Grattan's tireless campaigning the British parliament repealed Poynings' Law and much of the anti-Catholic legislation. 

Although suffrage was restored to Roman Catholics in 1793, the Irish parliament remained composed entirely of the Protestants of the established Church. The principles of the French Revolution found their most powerful expression in Ireland in the Society of United Irishmen, led by Wolfe Tone, Napper Tandy, and Lord Edward Fitzgerald, which instigated the rebellion of 1798. 

The peasantry rose in Wexford and, although insufficiently armed, made a brave fight. At one time Dublin was in danger, but the insurgents were defeated by the regular forces at Vinegar Hill. A French force of 1,100 landed in Killala Bay in Mayo but was too late to render effective assistance. The British prime minister, William Pitt, the Younger, thought that the legislative union of Great Britain and Ireland together with Roman Catholic emancipation was the only remedy for Roman Catholic rebellion and Protestant tyranny in Ireland. 

By a lavish use of money and distribution of patronage, he induced the Irish parliament to pass the Act of Union, and on January 1, 1801, the union was formally proclaimed. Owing to the opposition of George III, however, Pitt was unable to make good his promise of emancipation for Roman Catholics.