The road over the Wicklow Mountains was built by soldiers, and it shows. After the 1798 rebellion the British army cut a military road straight through the upland bog, so that troops could reach rebels who had been hiding in the glens, and the route from Laragh up to Sally Gap and over to Dublin still follows that logic: no hairpins, just long lines that climb where the ground allows. Glenmacnass is one of the points where the road leans over a drop, and the waterfall is the reason people pull in.
The stream comes off the high moor and falls in a long white run down a granite step into the valley below. You can hear it from the layby before you see it, and in spring or after heavy rain the volume is startling, a rope of brown-tinted water the colour of weak tea, stained by the peat it crossed. In a dry August it thins to a lace curtain, and the view back down the valley is the better show anyway. The glacier that carved this U-shaped glen left it wide and empty, with bracken, heather and bare hillsides and almost no trees.
The riding on the R115 is a gift when the weather behaves. The surface is generally decent, the corners open and readable, and the views come in big, quiet panels. Climbing from Laragh the road gets exposed, and the wind can hit hard enough to nudge you across the lane. Sheep wander onto the tarmac without ceremony, so keep the speed honest, and expect tour coaches and plenty of weekend cyclists. Go early on a weekday and you may have the glen mostly to yourself.
The closest towns in the data are Rathdrum and Valleymount, but for a real forecourt and a sit-down meal you are better off in Wicklow Town, or filling in the villages around Roundwood and Laragh before you commit to the high ground. Glendalough is close by, with its monastic round tower and two lakes, and makes a natural pairing with the waterfall for anyone who wants history alongside the scenery.
Bring a layer even in summer. The weather at this height changes quickly, and fog can sit on the moor while the coast is sunny.