Ireland and the United Kingdom are both part of the European Union (EU). The Crown Dependencies are not a part of the EU however do participate in certain aspects that were negotiated as a part of the UK's accession to the EU. Neither the United Kingdom or Ireland are part of the Schengen area, that allow passport-free travel between EU members states. However, since the partition of Ireland, an informal free-travel area had existed across the region. In 1997, this area required formal recognition during the course of negotiations for the Amsterdam Treaty of the European Union and is now known as the Common Travel Area.

The United Kingdom is part of what union?