I've always had a problem figuring out the difference between terms for British nationality -- most notably the difference between "British" and "English." I've also been pretty well stumped about whether everybody on any of the British Isles can be said to be from "the United Kingdom." Well, blogger Sam Hughes has finally answered these questions and more in a handy Venn diagram -- which I won't reproduce here at a legible size, to encourage you to view the diagram and read the accompanying explanation. Here's a sampler:
The UK's full name is "The United Kingdom of Great Britain and Northern Ireland". Citizens of the UK are called "British". One British person is called a Briton.
The ROI's full name is "The Republic of Ireland" (if you are speaking English) or "Ã‰ire" (if you are speaking Irish). Citizens of the ROI are called "Irish".
Irish citizens are not British citizens. British citizens are not Irish citizens. God help you if you forget this when you encounter an Irishman.
Check out the diagram and all your questions will be answered. (Well, you may have to print it and carry it with you, but still.) Also, as Hughes notes, this is actually a Euler diagram, but that hardly sounds as simple, does it?