evidence: The Republic of Ireland–United Kingdom border runs for 360 km between Lough Foyle and Carlingford Lough. It was established in 1921 when Ireland was partitioned but became an international b… The Republic of Ireland–United Kingdom border runs for 360 km between Lough Foyle and Carlingford Lough. It was established in 1921 when Ireland was partitioned but became an international border in 1922. While there are no customs posts, during the Troubles in Northern Ireland there were military checkpoints but these have since been removed. Now there are no restrictions on movement across the open border, though it was more heavily controlled during the period of conflict.