A 10km single-lane road loops a glacial valley beneath the Dartry Mountains, with vertical limestone cliffs, a ruined barytes mill and the legendary cave of Diarmuid and Gráinne.
A short woodland walk weaves through the stone foundations of the 19th-century mill, which processed barytes until the mid-20th century, past native trees, streams and waterfalls - a good picnic spot. There is also a sculpture and fairy trail behind the mill.
The easiest way to see the horseshoe: a single-lane road runs clockwise round the valley floor beneath the Dartry Mountains, taking about 25-30 minutes without stops. Start at the Barytes Mill car park and drive clockwise for the best first view of the cliffs.
Two routes share the valley: the 13.5km Benwiskin Trail from the Benwiskin Centre (moderate, about 2.5 hours, only 40m of ascent) or the shorter 9.1km Mill Site circuit from the Barytes Mill car park. Neither is waymarked on the ground, so navigate by the road signs.
Jaw-dropping vertical limestone cliffs rise from the valley floor. The cave mouth high on the eastern cliff is said to be the highest of its kind in Ireland; view it from the roadside beside the old schoolhouse. Do not climb - the approach crosses private land and is a genuine fall risk.
A signposted gravity hill where, with the car in neutral, you appear to roll uphill. It is a trick of perspective, and good fun.
A ruined 19th-century schoolhouse beside the road is a fine photo stop, and a small megalithic dolmen ringed by stones sits on the eastern side of the loop.
Watch for mountain hares, hovering kestrels, peregrine falcons and occasional red deer. Spring brings bluebells, primroses and wild garlic to the hollows; autumn turns the bracken and heather copper.
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