The island's only land border is between the Republic of Ireland (RoI) in the south and Northern Ireland (NI) - which is part of the United Kingdom - in the north east of the island. Its 360kms (224m) stretch from Lough Foyle on the northern edge of the island to Carlingford Lough in the east, on the Irish Sea. In common with many internal borders in the European Union, it is very inconspicuous and open by world standards.You must drive on the left hand side of the road on both sides of the border. Beware of the change in speed limits when crossing. All speed limits in the Republic Of Ireland are in kph whilst in Northern Ireland they are in mph. TIP: Before crossing the border to NI, fill your petrol tank as fuel is cheaper in the RoI.