1. England and Wales were the first to become united
2. The UK as such was formed by the Act of Union of the Parliaments of England/Wales with that of Scotland in 1707/1708
3. Ireland was colonized over the course of centuries
4. In 1800/1801 the Irish Parliament was absorbed by the UK Parliament
5. After World War I, the Republic of Ireland fought a war for independence
6. Northern Ireland voted to remain in the UK