What you see there is all of the British Isles. You've got England, obviously, and Scotland and Wales. The Hebrides, the Shetlands, the Orkneys, the Isle of Wight, the Isles of Scilly and the Isle of Man are all included in the UK. So are the Channel Islands, Jersey and Guernsey, even though they're technically closer to France than England.
Now, on the left is the entirety of Ireland. Way back when, that was all one country, under Irish rule. Then the Tudors came along and took it for themselves, getting through all the rebellions and such to keep it. Then, when the British Empire was established, Ireland was included, which they weren't happy about. When the UK was founded, Ireland as a whole was also included.
But then, around the late 1800s-early 1900s, the whole age-old religious argument between Protestants and Catholics started to become more prevalent in Ireland, and it looked like a war was brewing. So the UK split it in 1920, into Northern Ireland and the Republic of Ireland. Northern Ireland is the region that was then Protestant, so joined the Protestant UK. The Republic of Ireland was the Catholic area, which is why it has since become closely related to America, which was heavily Catholic back then too (You could argue some parts still are heavily Catholic, to be fair).
Get the idea?