On 1 January 1801 the two kingdoms of Great Britain and Ireland were united, as The United Kingdom of Great Britain and Ireland. The kingdoms either side of the Irish Sea had been united in effect since the Middle Ages, when King Henry II conquered the warring kingdoms and chiefdoms of Ireland, though subsequent kings had found the need to reconquer parts of the island, but from 1801, Ireland was not a subjected land but an equal part of the one, united kingdom.
The legal union was effected by two acts of Parliament, passed in the British and Irish parliaments respectively, which Acts were later given the short titles:
The full, original texts are provided in the links.