Why is southern Ireland not part of UK?

Related questions

1. Is Ireland under British rule?
2. Is Ireland independent of England?
3. Is English widely spoken in Ireland?
4. What is the peak tourist season in Ireland?
5. How much is a taxi from Dublin Airport to the city?
6. When did Ireland leave the UK?
7. Can I go to UK with Irish visa?
8. Do I need shots to go to Ireland?
9. Is Irish the same as Gaelic?
10. Is it safe to travel to Ireland?

Tags

1. #united
2. #previously
3. #officially
4. #kingdom
5. #independent
6. #client
7. #indepenence
8. #since
9. #gained
10. #status
11. #prior
12. #southern
13. #republic
14. #state
15. #other
16. #british
17. #northern
18. #ireland