Britain and Ireland
The British and Irish Isles refer to the islands of Great Britain, Ireland and nearby islands. They are located in Europe.
The Channel Islands are sometimes included in the term "British Isles" because they are a British crown dependency. However, they are located off the coast of Normandy, France, and are not geographically part of the British or Irish Isles.
The Common Travel Area
The United Kingdom, Northern Ireland, Ireland and other adjacent territories in the United Kingdom, maintains a common travel area, somewhat akin to the Schengen Area on continental Europe. Visitors who have been checked through immigration in e.g. England wouldn't normally have to go through immigration procedures when continuing a journey in Ireland etc. The foundations of this agreement has been somewhat watered out since Ireland initiated spot checks at it's borders some years back.
Throughout this region, traffic drives on the left. In the United Kingdom, and its Crown Dependencies Speed Limits are expressed in Miles per hour. In the Republic of Ireland speed limits are expressed in Kilmometers per hour.