Overview
The Gleniff Horseshoe is a single-lane road circuit on the northern edge of the Dartry Mountains in County Sligo, running through a U-shaped glacial trough with vertical limestone cliffs on either side. You can drive it, 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.
The loop is a shared public road. Give walkers, cyclists and oncoming vehicles room, and keep clear of farm entrances if you stop. The cave high in the cliff is a view from below: its approach crosses private land and is dangerous even for experienced walkers.
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 separate woodland path beside the stream. It is a better choice than the full road walk if you only want a short stop.
Diarmuid and Gráinne’s cave
The dark mouth high on the eastern cliff is Diarmuid & Gráinne’s Cave. 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 Sligo Walks reports rescues from this approach. Stay below and use the photograph to see where the cave sits in the cliff.
Walking the loop
The narrow lanes need care when a farm vehicle comes the other way, and there is no separate footpath, so walkers share the carriageway. The Sligo Walks guide describes the Benwiskin Trail, a moderate road walk around the horseshoe that starts near Ballintrillick; check the guide for the current route, length and what to expect underfoot before you set off.
- Benwiskin Trail: the longer option, looping the horseshoe on country roads. Check the Sligo Walks guide for length and timing.
- Mill site woodland path: a short stroll beside the stream and through the woodland at the old mill, closer to the water. Better for a relaxed stop or a picnic than the full road walk.
For a managed woodland circuit away from the valley road, Coillte’s Gortarowey trails include the 5.5km Benbulben loop. If you prefer the coast, head to Mullaghmore and start from the village seafront.
Other stops on the loop
- Magic Hill: a signposted gravity hill where the road appears to run uphill. It is a trick of perspective and good fun, but pull in only where it is safe and do not block the lane.
- 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: The loop lies north of Sligo town, reached from the N15 near Cliffoney and close to Mullaghmore. Watch for the road signs to Gleniff rather than expecting a signed trailhead, and check your route before you go.
- Parking and facilities: Use signed or lawful parking and keep farm gates and the carriageway clear. Do not count on 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 public road and the signed 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.
- 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 has 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.