How did Ireland become a part of the UK?

From the 12th century, much of Ireland was effectively under English control, and, between 1541 and 1801, the Kingdom of Ireland was a separate kingdom within the British Empire. This arrangement ended in January 1801, when the Act of Union united Ireland and Great Britain into the United Kingdom of Great Britain and Ireland by merging their parliaments and governments.

Following the Irish War of Independence of 1919-21, most of the island seceded from the United Kingdom and became the Irish Free State, a dominion within the British Empire, in accordance with the 1921 Anglo-Irish Treaty. Northern Ireland, which had a Protestant majority, chose to remain part of the United Kingdom.