Northern Ireland is a constituent country of the United Kingdom, which is on the island of Ireland. Northern Ireland shares an open border to the south and west with the Republic of Ireland. The centrepiece of Northern Ireland's geography is Lough Neagh, the largest freshwater lake both on the island of Ireland and in the British Isles. A second extensive lake system is centred on Lower and Upper Lough Erne in Fermanagh. The largest island of Northern Ireland is Rathlin, off the north Antrim coast.