Overview
The Gleniff Horseshoe is a 10km single-lane loop on the northern edge of the Dartry Mountains in County Sligo, running clockwise through a U-shaped glacial trough with vertical limestone cliffs on either side. You can drive it in about 25 minutes, cycle it, or walk it, and the honest answer is that the drive gives you most of what the walk does. Do it both ways round if you have time; the view of Benwiskin, Truskmore and Donegal Bay changes depending on which direction you face.
How the valley formed
The trough is the work of the last Ice Age. Glaciers more than a kilometre thick ground through the Dartry range, carving the deep U-shaped valley that defines the horseshoe; the cliffs are limestone of the Dartry Formation. The ice retreated around 10,000 years ago, leaving the sheer rock faces and the flat pasture floor you drive across today.
The barytes mill
Beside the river stand the ruins of the Gleniff Barytes Mill. From the late 19th century it processed barytes, a heavy white mineral used in paint, plastics, pottery and, later, as the barium in X-ray procedures. A private railway carried the product down to Mullaghmore harbour for shipping until the mid-20th century, and the mill itself burned down in the 1920s. The stone foundations and water channels remain, with a short woodland trail through them, and the car park here is the main starting point for walkers.
Diarmuid and Gráinne’s cave
The dark mouth high on the eastern cliff is Diarmuid & Gráinne’s Cave, said to be the highest cave of its kind in Ireland. In the Fenian tale, the lovers Diarmuid and Gráinne hid here from Fionn mac Cumhaill; Diarmuid was later killed by a wild boar on Benbulben.
One firm warning: do not attempt the climb. The approach crosses private land and steep, unstable ground, and the Sligo-Leitrim Mountain Rescue Team has been called out to injured visitors more than once. Admire it from the valley floor.
Walking the loop
The road is quiet and largely traffic-free, though the narrow lanes need care when a farm vehicle comes the other way. Two waymarked walks share the valley, and it’s worth knowing before you set off that neither is actually waymarked on the ground: the official Sligo Walks route lists waymarking as ‘none’, and you navigate by the road signs.
- Benwiskin Trail: the longer option, 13.5km, graded moderate, allow around 2.5 to 4 hours depending on your pace. It starts from the Benwiskin Centre (roadside parking and an information board) and loops the horseshoe on quiet country roads with only about 40m of ascent.
- Mill Site walk: a shorter 9.1km circuit of roughly three hours from the Barytes Mill car park, closer to the stream and the woodland. Better for a relaxed half-day or a picnic.
Other stops on the loop
- Magic Hill: a signposted gravity hill where, with the car in neutral, you appear to roll uphill. It’s a trick of perspective, and it’s good fun.
- Dolmen: a small megalithic tomb ringed by stones on the eastern side of the loop, a reminder of how long people have used this valley.
- The cliffs: the towering limestone faces drop vertically into the valley, their sedimentary layers stacked plainly for anyone to read.
Wildlife & Flora
The horseshoe sits in a transitional zone between lowland pasture and upland moorland, supporting a varied ecosystem. Mountain hares are a common sight on the open slopes, while kestrels hover over the grassy verges. Peregrine falcons use the limestone cliffs as nesting sites, and red deer are occasionally spotted along the tree lines. In spring, the hedgerows and damp hollows fill with bluebells, primroses, and wild garlic. Autumn brings a carpet of bracken and heather turning copper and purple, making it a particularly photogenic time to visit.
Practical tips for visitors
- Getting there: From Sligo town, drive north on the N15 to Cliffoney, about 22km, then follow the signs roughly 6km to the loop. It’s about 15 minutes from Mullaghmore and 25 minutes from Bundoran. Watch for the road signs to Gleniff Barytes Mill rather than expecting a signed trailhead.
- Parking and facilities: Free parking at both the Benwiskin Centre and the Barytes Mill site. There are no toilets or shops on the loop; sort that in Cliffoney or Mullaghmore before you set out.
- What to wear: Upland weather turns fast, and the higher sections offer little shelter. Even on a mild day, bring waterproofs, sturdy shoes and a windproof layer.
- Safety: Stick to the road and the marked routes. The cave approach is on private land and a genuine fall risk. If you go off-route, tell someone your plans and carry a charged phone; in an emergency, the Sligo-Leitrim Mountain Rescue Team covers this ground.
- Best time to go: Spring and early autumn give the most reliable weather, with bluebells, primroses and wild garlic in the hollows in spring and the bracken and heather turning copper in autumn. Winter daylight is short and the road can ice over.
Nearby
- Mullaghmore: The harbour village is about 15 minutes away, with a sandy strand, a headland drive and the outline of Classiebawn Castle.
- Benbulben Forest Walk: A short loop around the southern flank of Sligo’s flat-topped mountain, with woodland trails and coastal views, a little to the south.
- Ballygilgan Nature Reserve: A coastal reserve near Lissadell, south-west along the coast, good for waders and migratory geese in winter.
Come after a wet spell and you’ll get the cliffs at their most dramatic, with the run-off streaming down the limestone; come in a dry May for the flowers and the easiest driving.