The Northern Ireland Peace Process has led to a number of unusual arrangements between the Republic of Ireland, Northern Ireland and the United Kingdom. For example, citizens of Northern Ireland are entitled to the choice of Irish or British citizenship or both and the Governments of Ireland and the United Kingdom consult on matters not devolved to the Northern Ireland Executive. The Northern Ireland Executive and the Government of Ireland also meet as the North/South Ministerial Council to develop policies common across the island of Ireland. These arrangements were made following the 1998 Good Friday Agreement.

What has caused several uncommon arrangements between the various states in the United Kingdom?