After the Good Friday Agreement was concluded, customs and security controls between the Republic of Ireland and the United Kingdom were removed (From Wikipedia). The Irish border is one of the major problems in the Brexit negotiations. If the UK leaves the EU Customs Union, it
may will almost certainly be inevitable to reintroduce customs controls at the UK-Republic of Ireland border.
Does the Good Friday Agreement necessarily mean that such controls must not take place, or can customs controls exist without violating the Good Friday Agreement?