Slieve Donard (ˌsliːv_ˈdɒnərd ; ) is the highest mountain in Northern Ireland and the wider province of Ulster, with a height of . 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, and 7th highest on the island.
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. Also on the summit are the remains of two ancient burial cairns, one of which is the remains of the highest known passage tomb in Ireland. In Irish mythology the mountain was associated with the mythical figures Boirche and Slángha. It was later associated with, and named after, Saint Donard, who was said to have made the summit his hermitage. Up until the 1830s, people would climb the mountain as part of a yearly pilgrimage, which may have originally been a Lughnasadh (harvest) ritual. Royal Engineers camped on the summit for four months in 1826 as part of the Ordnance Survey's Principal Triangulation.
Slieve Donard sits at the northeastern edge of the Mournes, overlooking Newcastle and Dundrum Bay. It has two lesser summits on the seaward side – Millstone Mountain (460 m), and Crossone (540 m). Two glens separate Slieve Donard from the neighbouring mountains of Slieve Commedagh (to the northwest) and Chimney Rock Mountain or Slieve Neir (to the south). Slieve Commedagh, at , is the second-highest of the Mourne Mountains.
The mountain is an easy climb although the path is very eroded at places. The most popular route begins at Donard Forest on the north side of the mountain and follows the Glen River and then the Mourne Wall to the summit. In recent years a stone path has been made on the steepest parts of the mountain. The summit has views of the coast, and as far afield as Belfast Lough, north, Dublin Bay, south, and the Isle of Man.
On the summit are the remains of two prehistoric burial cairns.