Although the Irish Parliament had won its supremacy, opposition to British rule remained. In 1790 the Society of United Irishmen - mainly Protestant - was formed in Belfast. Its founder was Theobald Wolfe Tone.
This discontent led the British Parliament to fear that the two kingdoms were drifting further apart.
Legislative – or parliamentary – union between Ireland and Britain had been mooted in the early 18th century and was revived when the United Irishmen rebelled in 1798.
Mindful of the need to maintain the Protestant Ascendancy William Pitt, the British Prime Minister, pursued the idea.
On 22 January 1799 the Irish House of Commons was tentatively asked to consider some form of union. Two days later the motion was defeated, although it later passed in the Irish House of Lords.
People as well as parliaments
Pitt realised that if union was to be peaceful and long-lasting, it had to be a union of peoples as well as parliaments, which is why he pledged that Catholic voting rights would follow soon after. In the British Parliament, proposals for union passed easily.
In Ireland, meanwhile, the British government gradually sought to change public and political opinion. Sympathisers took seats in the Irish House of Commons, while peerages and pensions were promised to landowners in order to win support.
Finally, on 28 March 1800, both houses of the Irish Parliament accepted the terms of union.
These were then laid before the British Parliament and, following minor amendments, passed through both Houses.
Royal Assent was granted on 1 August, and the Act of Union came into force on 1 January 1801, ending five centuries of separate Irish parliamentary history.