Above: The iconic Great Cairn and stone shelter on the summit of Slieve Donard

Elevation: 850m / 2970ft
Slieve Donard is the highest peak in the Mourne Mountains, and also the highest peak in the wider province of Ulster. For this reason it by far the most popular peak in the Mournes among hikers, especially during the summer months.
There are two main routes to the summit; the first and, in my opinion, the best route begins in Newcastle, follows the Glen river, ascending steeply through Donard Forest before becoming more gradual as the forest gives way to the open valley (marked in red).
A steep section at the end of the valley leads onto the saddle between Slieve Donard and Slieve Commedagh. At this point the Mourne wall can be followed all the way to the summit. The Mourne wall will also provide shelter much of the time on the north side as the wind usually blows from the south. There is an established stone path on the north side.
The second route begins at Bloody Bridge between Newcastle and Annalong (marked in blue). The route follows a rough track all the way up to a disused quarry. Above the quarry lies the bog of Donard which needs to be negotiated before a right turn is taken at the Mourne wall and followed all the way to the summit.
The two main routes to the summit of Slieve Donard

The Glen River route to Donard summit, beginning with Donard Forest
As with all of the peaks in the Mournes, the views can be fantastic on clear days. The Isle of Man, Scotland, County Tyrone on the west side of Lough Neagh and far into County Louth on the south of Carlingford Lough.

The view looking north from Slieve Donard Summit toward Newcastle
The View looking west from Donard Summit across Slieve Bearnagh
Despite an elevation of less than 1000 metres, the weather conditions on Slieve Donard summit, and many of the other peaks, can be considerably different than at sea level. Notably wind speed and temperature. The temperature on the summit can be as much as 7 degrees Celsius lower than that at sea level, with a wind speed up to 3 times stronger. This combination can make Slieve Donard summit, and other peaks, a dangerous place. Especially in the winter months.
Some severe winter and snow conditions on Slieve Donard, taken two separate weekends in March 2013. Note the depth of the snow against the Mourne Wall and also the effect of the strong wind in shaping the ice and snow.

Severe snow conditions on Slieve Donard Summit, March 2013. Around the same time as the photos above.

An example of the strong wind that often exists on Slieve Donard summit. On this day it was from the west. December 2014.
Check out my tips and safety information for the hills here