Ireland shares a land border with the United Kingdom on the island of Ireland. More precisely, the Republic of Ireland borders Northern Ireland, which is part of the United Kingdom.
The border was created in 1921 under the Government of Ireland Act, which partitioned the island into Northern Ireland and Southern Ireland. It became an international frontier in December 1922, when Northern Ireland opted out of the newly established Irish Free State. The modern Irish state is constitutionally named Ireland, although Republic of Ireland is its legal description.
The border stretches roughly 500 kilometres from Lough Foyle to Carlingford Lough and is famously irregular, with around 270 public road crossings. It is often difficult to identify on the ground because of the Common Travel Area and the post-1998 peace process. Brexit made it an external EU border, but agreements continue to aim for an open land frontier.