But there is a twist here: Great Britain and the United Kingdom are not the same thing. Great Britain is just the name of England, Scotland, and Wales.
Then in 1801, Ireland was forcibly included in this union, and it became the United Kingdom of Great Britain and Ireland.
But the Irish people did not want to live under the UK. After years of protests, fighting, and war, in 1922 a large part of Ireland became independent, which is now the Republic of Ireland.