Overview
Fifteen kilometres southwest of Dublin city, Glenasmole feels a good deal further out than that. The valley is defined by the twin Bohernabreena reservoirs, set between the Dublin Mountains and the edge of Wicklow National Park. It does serious modern work, supplying drinking water to roughly 35,000 South Dublin households, but its marked trails, open meadows and ruins make it a reliable escape for walkers, birdwatchers and families.
History and mythology
The valley dates to the last Ice Age, when retreating glaciers carved its U-shape. The name carries two possible meanings: Gleann na Smól(aigh), ‘valley of the thrush’, or Gleann na Smóil, ‘valley of the burnt patch’, a nod to the legendary burning of the Dá Derga hostel in early Irish myth. Nearby Bohernabreena comes from Bóthar na Bruíne, ‘the road to the mansion’, tying the place to old tales of hospitality and chieftains.
The area is woven into the lore of Fionn Mac Cumhaill and Na Fianna. Local tradition holds that Fionn hunted red deer and the now-extinct great Irish elk through these woods, feasting afterwards on the summit of Seefin, which means ‘the seat of Finn’. Another legend puts the poet-warrior Oisín here: returning from Tír na nÓg, he leant from his white horse to help locals lift a boulder, touched Irish soil, and aged 300 years in an instant.
There is tangible history too. The Bohernabreena Waterworks were built between 1883 and 1887 to tame the flood-prone River Dodder, power local mills and supply clean water to Rathmines. The system still runs: the upper reservoir collects clear, non-peaty water, the lower holds the more peaty runoff, and a stone-lined channel links the two before feeding the treatment works at Ballyboden, 7.5 km away. Near the lower loop, the ruins of St Anne’s Graveyard go back to at least the 10th century. Closed to burials because of its proximity to the water supply, the site still hosts an annual Mass each July and keeps a ‘Strangers’ Corner’ for those buried away from home.
What to see and do
The reservoir walks
Two loop trails suit different paces and time budgets, both starting from accessible car parks.
Lower Reservoir Loop (short) Around 4 km and 1 to 1.5 hours, this is the popular choice for casual walkers and families. It begins near the new Upper-Lake car park. Head clockwise from the trail entrance so you don’t miss the main sights. The path hugs the water’s edge, runs through mixed woodland and brings you to the ruins of St Anne’s Graveyard before looping back.
Long Loop The 8.5 km long loop takes about 2 hours. Starting from the original Bohernabreena North Car Park, it opens with a quiet stretch along a country road before dropping to the lower reservoir, then climbs gently to the upper reservoir and follows the historic stone channel home. The opening road section has no footpath; the rest is a gentle, well-trodden trail that most fitness levels will manage.
Dublin Mountains Way – Stage 8
If you are after distance, join Stage 8 of the 42.6 km Dublin Mountains Way. This 6 km segment runs from Glenasmole to Tallaght, following the River Dodder through Kiltipper Park and past residential greenways. Look for the national yellow hiker symbol and the directional arrows that waymark the route.
Flora and fauna
Designated an Area of Special Conservation, the valley supports a delicate ecosystem on its damp, peat-rich soils and open water. Spring brings wild orchids, including early purple and frog orchids, alongside extensive molinia meadows. Its petrifying springs leave striking mineral deposits as the calcium-rich water evaporates.
The reservoirs and woodlands reward birdwatchers year-round. Kingfishers, herons and moorhens work the water’s edge, while meadow pipits and skylarks are common in the grasslands. Winter brings waders such as curlew and snipe. Anglers should note that trout fishing is permitted only with a fly rod and a valid permit from a local angling club.
Practical information
Parking and access
The valley is open 24 hours and free to enter, but the parking needs thought. The original car park at the north end has just 15 free spaces and fills quickly at weekends. Roadside parking is strictly enforced and cars are frequently towed. The safer bet is the new Upper-Lake car park, which charges €5, offers secure parking and has toilets. A weekend coffee stall, Goughie’s, sets up nearby.
Public transport
Driving isn’t the only way in. On Saturdays the Local Link Kildare South Dublin Route SD03 stops at the Upper Reservoir Gate and the Glenasmole Community Centre. Check the TFI Live app for real-time times, as rural services can be thin.
Amenities and accessibility
Toilets are at the Upper-Lake car park. The Glenasmole Community Centre, roughly 9 km from Tallaght, has tea, coffee and further facilities Monday to Friday (9am–5pm) and weekends (9am–5pm). Dogs are welcome on all trails but must be kept on a lead throughout to protect ground-nesting birds and livestock. The paved path from the Upper-Lake car park to the reservoir edge is flat and wheelchair accessible, though the loop trails themselves are uneven and occasionally boggy.
Visitor tips
- Arrive before 10am at weekends to get parking and beat the crowds.
- Wear sturdy, waterproof footwear; the trails turn muddy and slippery after rain.
- Mobile signal can be unreliable in the valley, so download offline maps or carry a physical trail guide.
- Bring a reusable water bottle and a light snack; facilities are limited outside the car park and community centre.
Nearby highlights
Glenasmole makes a handy base for the wider Dublin Mountains:
- Cruagh Wood (20-minute drive) – the Slí na Sláinte loop, with views over Dublin from 522 m.
- Blessington (25-minute drive) – a lakeside town with the Blessington Greenway and easy access to Russborough House.
- Ticknock (30-minute drive) – the Fairy Castle passage tomb and sweeping views over Dublin Bay.
Late spring catches the wild orchids in bloom; autumn turns the deciduous woods and sets them reflecting in the reservoirs. On the short loop, take the second entrance at the trailhead and stick clockwise so you pass St Anne’s Graveyard before the car park.