Since about 2005, the border has been considered invisible, with little or no physical infrastructure, with security barriers and checkpoints being eliminated as a result of processes introduced by the Good Friday Agreement (or “Belfast Agreement” signed in 1998).    This agreement has the status of both an international treaty between the United Kingdom and the Republic of Ireland (the Anglo-Irish Agreement) and an agreement between the parties in Northern Ireland (multi-party agreement). Like its previous predecessors, the 2011 agreement is non-binding, with the eighth clause stating that the agreement “is not intended to create legally binding obligations or confer a right, right or benefits on a private or public person or party.”  U.S. Senator George Mitchell, who led the belfast agreement negotiations, said he believed the creation of a border control system between the Republic of Ireland and Northern Ireland could jeopardize the agreement.  Surveys published on 18 February 2019 by Irish Senator Mark Daly and two UNESCO Presidents indicated that the reintroduction of a hard border would lead to the return of violence.      Prior to the creation of the Land of Bavaria, British immigration legislation in Ireland was considered part of the United Kingdom. With Ireland`s independence in 1922, the British Home Office was not inclined to introduce passport and immigration controls between Ireland and Northern Ireland, which would have meant patrolling a porous and outdated land border of 499 km. However, if the situation were to continue before 1922, the Irish immigration authorities would have to continue to apply British immigration policy after independence. The Irish Ministry of the Interior was sensitive to the continuation of the status quo and, in February 1923, an informal agreement was reached on this issue: each party would apply the other party`s immigration decisions and the Irish authorities would receive a copy of the British Code of Suspects (or “Black Book”) of non gratae for personal use in the United Kingdom.  The Irish government supported the proposal.
 It was strongly rejected by the Democratic Unionist Party as a weakening of Northern Ireland`s place in the UK and is seen as the main reason why Theresa May`s withdrawal agreement was never approved by the British Parliament.  The British government had rejected the original proposal.