In 1797 William Pitt appointed Lord Castlereagh as his Irish chief secretary. This was a time of great turmoil in Ireland and in the following year Castlereagh played an important role in crushing the Irish uprising. Castlereagh and Pitt became convinced that the best way of dealing with the religious conflicts in Ireland was to unite the country with the rest of Britain under a single Parliament.

The policy was unpopular with the borough proprietors and the members of the Irish Parliament who had spent large sums of money purchasing their seats. Castlereagh appealed to the Catholic majority and made it clear that after the Act of Union the government would grant them legal equality with the Protestant minority. After the government paid compensation to the borough proprietors and promising pensions, official posts and titles to members of the Irish Parliament, the Act of Union was passed in 1801.

George III disagreed with Pitt and Castlereagh's policy of Catholic Emancipation and after the passing of the Act of Union approached Henry Addington to become his prime minister. When he heard what had happened William Pitt resigned from office and was therefore unable to deliver religious equality in Ireland.

In 1823 Daniel O'Connell, Richard Lalor Sheil and Sir Thomas Wyse formed the Catholic Association. The organisation campaigned for the repeal of the Act of Union, Catholic Emancipation, the end of the Irish tithe system, universal suffrage and a secret ballot for parliamentary elections. The Catholic Association grew rapidly and in 1829 Sir Robert Peel, Duke of Wellington and other leading members of the government began arguing for reform. They warned their Conservative colleagues that here would be civil war in Ireland unless the law was changed. In 1829 the British Parliament passed the Roman Catholic Relief Act, which granted Catholic Emancipation. However, despite Daniel O'Connell forming the Repeal Association, the Act of Union remained in place.