Acts of Union 1800

The Acts of Union 1800 (or sometimes Acts of Union 1801) (Irish: "Acht an Aontais 1800") is used to describe two Acts are meant. Their official titles are the Union with Ireland Act 1800 (1800 c.67 39 and 40 Geo 3), an Act of the Parliament of Great Britain, and the Act of Union (Ireland) 1800 (1800 c.38 40 Geo 3), an Act of the Parliament of Ireland.
These two Acts merged the Kingdom of Ireland and the United Kingdom of Great Britain, (being itself a merger of the Kingdom of England and the Kingdom of Scotland under the Acts of Union 1707), to create the United Kingdom of Great Britain and Ireland.
The Union Flag.
The flag created as a consequence of the merger combined the flags of England and Scotland with "St Patrick's Cross" to represent Ireland. Wales is not included on the Union Flag, as when the original Union Flag was devised Wales was a part of the Kingdom of England.