Slieve Donard via Black Stairs, returning via Crossone and Millstone Mountain

If you want to avoid the crowds going up Donard then going up via the Black Stairs is a good alternative. For those who don't know the Mournes well, here's a guide to the route.

1. Glen River to top of Donard Wood
From Donard car park head up the broad track to the right of the pretty Glen River.
After the first bridge the main track continues on the left bank of the river (but I headed up through the woods on the right).
After the second bridge the main track switches back to the right hand bank.
After the third bridge the track continues on the right and shortly afterwards you come out of the wood into open countryside.

2. Top of Donard Wood to Slieve Donard.
Look across to the Glen River a short distance upstream and you'll see the circular building which is the Ice House. Take a narrow path which angles down to the river to a place just near the ice house where it is easy to cross.
Climb up beside the ice house and find the path which goes on the right hand side of the stream (a tributary of the Glen River). If you look up the stream you'll see the Black Stairs gully in the hillside ahead which looks like a stepped series of waterfalls. Follow the path towards it which will bend to the right before the gully and leads you up by a route which avoids the cliffs. The path is quite steep, and you'll need to use your hands in a couple of places, but is not at all dangerous.
Once you're above the rocky bluff, head back towards the river, reaching it above the waterfalls, and turn right uphill. From here on it's a fairly steep but straightforward climb up to the summit, mostly following a path.

3. Slieve Donard to Bloody Bridge quarry.
The Mourne wall at the top does a right angle turn, and descending you go straight on beside the wall, following your direction of approach to the summit (roughly south-west). I think it's best to cross the stile and go down to the right of the wall, because the views towards the central Mournes Mountains are stunning.
At the bottom, cross the stile and descend the broad track which goes eastwards down by the Bloody Bridge River, arriving at the quarry after about 800m.

4. Bloody Bridge quarry to Crossone
If you follow the GPS track, you'll see that I walk down the road on the right hand side of the river, cross the river at a ford, and then go up a zig-zag path to a little ruined house half way up Crossone Mountain.
As an alternative you can walk down the rough path on the left-hand side of the river (which avoids a little retracing of steps), but the best way might be to cut across country aiming for the waypoint I marked as "End of Detour". At the top of the zig-zag I saw this path heading back towards the quarry and followed it for some time, until it seemed to disappear, and then I turned back (at the "End of Detour" waypoint).
From the little ruined house to the top of Crossone there doesn't seem to be any path, so you just have to make your own way up the stone slabs and heathery slope. At the top though you're rewarded with a little stone windbreak inside which you can sit and enjoy the views to the sea.

5. Crossone to Granite Trail quarry
From the summit of Crossone you have to contour around the flank of Donard, around a large bowl, to get to Millstone Mountain. There are no paths on the stretch, and you're unlikely to meet anyone else!
At the summit of Millstone Mountain continue northwards along the line of least slope, as the terrain drops you into a small valley and you cross a small stream. You pick up a path – this is another route up Donard – which leads you down to the top of Donard Wood just beside the quarry at the top of the Granite Trail. (In fact, if you wanted a much shorter return route from the summit of Slieve Donard you could go back down the way you came up and then edge towards the right to get to this path).

6 Granite Trail quarry to car park
From the gate at the bottom of the path it's an easy walk along the forest road to the third bridge on the Glen River which you crossed on the way up, and then follow the track back down to the car park.

Overall I've marked the trail as Difficult because of the length and elevation, and the amount of walking on rough ground rather than paths.