Slieve Donard is the highest mountain in Northern Ireland, the highest in Ulster and the seventh-highest in Ireland, with a height of 850 metres. The highest of the Mourne Mountains, it is near the town of Newcastle on the eastern coast of County Down, overlooking the Irish Sea. It is also the highest mountain in the northern half of Ireland. The Mourne Wall – built in the early 20th century – runs up the western and southern slopes of the mountain, joining a small stone tower at the summit.