On 1 August 1800, the Parliament of Ireland approved the Act of Union which, in conjunction with the earlier approval of union by the Parliament of Great Britain on 2 July 1800, united Ireland with Great Britain and established the United Kingdom of Britain and Ireland on 1 January 1801.
While Ireland and Great Britain had been united in a personal union under the monarch of Great Britain and Ireland since 1603, political union would come much later.
The Parliament of Great Britain sought closer union with Ireland after the French Revolution of 1789 and the Irish Rebellion of 1798. It was fearful that the majority Roman Catholic population obtained the right to elect Catholics into the Irish Parliament that such a Catholic Parliament would attempt to break away from Britain and turn toward France. A united kingdom would prevent any attempt by Ireland to abandon its connection to England and Scotland.
Indeed, in order to get Irish Catholic support for Union which would abolish the separate Irish Parliament for a united Parliament in Britain, the Catholics were promised Emancipation which would allow Roman Catholic members of Parliament. However, after the passing of Union King George III refused to permit Catholic Emancipation on the grounds that it would be a violation of his oath to defend the Church of England. So, Irish Catholics could elect members of Parliament but no Irish Catholic could take a seat in the Parliament.
Catholic Emancipation would not be achieved until 1829.