Ireland and the UK. Literally two inseparable countries. Ireland's only neighbor is UK, and UK's only neighbor is Ireland. Now, sure, UK does have an underwater bridge connecting to France, but we're talking about natural land. The city that makes Ireland famous is Dublin. The city that makes UK famous is London. They have a huge population difference, with Ireland having approximately 5 million people and the UK having about 65 million people. The two countries also have a big size difference, with Ireland being approximately 70 thousand square kilometers, and the UK having around 244 thousand square kilometers. There is also a wide number of territories UK has, including Montserrat, Reunion, Anguilla, Jersey, Guernsey, Bermuda, Gibraltar, Isle of Man, and many more. Ireland, on the other hand, has none. (Sorry if I'm making Ireland look bad.)