The mountain and the lake
Tonelagee means ‘backside to the wind’ in Irish (Tóin le Gaoith) and at 817 metres the summit earns the name. The reason most people know it is Lough Ouler, a glacial lake shaped almost exactly like a heart, held in a corrie under the northeast ridge. Its prominence of 202 metres makes it a Marilyn, and it also appears on the Arderin and Vandeleur-Lynam lists.
The heart shape is visible from above. The lake sits at around 600 metres below a cliff face rising close to 200 metres, and the straight-down-on-the-heart photographs you will have seen are usually taken by drone. Low cloud can hide the lake and make the surrounding terrain difficult to navigate. The summit itself is a flat trig point.
The mountain lies within Wicklow Mountains National Park, a landscape of heather plateaux, glacial valleys and old stone monuments, and it sits on the central spine that runs from Kippure toward Lugnaquilla.
History and landscape
The name has turned up in old records in variant spellings such as Tonelegee. The rock underfoot is adamellite, a coarse granite studded with large microcline crystals, which gives the upper slopes their weathered, light-catching texture.
The shape of the place is glacial. During the last ice age, moving ice gouged out the deep amphitheatre that now holds Lough Ouler, and the cliff on its northeastern side frames the lake in a natural bowl. Locally the lake is also called Lough Iolar, ‘lake of the eagle’.
Walking in the area
The mountain is exposed and has no waymarked summit trail. If you plan to hillwalk, check your intended route with Wicklow Mountains National Park beforehand, including access and weather conditions. Keep clear of the rim above the lake, turn back if visibility deteriorates and never attempt a river crossing in high water. Dogs must stay on leads in the national park.
For a signed route with lake and mountain views, use the national park trails at Glendalough. The information office by the Upper Lake has maps and advice on routes for different levels of experience.
Practical information
- Navigation: there is no signposting on the mountain, so carry a GPS device, offline mapping or a paper map, and check the national park’s maps and weather advice first.
- Facilities: none on the mountain.
- Seasonality: the ground is soft and boggy after rain, and winter on the upper slopes calls for ice axe, crampons and navigation skills.
- Safety: loose boulders, steep ground and open bog; good footwear and layers matter, and the weather turns quickly on this open, exposed ground.
Nearby
- Glendalough – a monastic settlement in a glacial valley with the national park’s signed trails.
- Avoca Valley – river walks and craft heritage.
- Blessington Lakes – angling and watersports.
- Aghavannagh – a quiet village with access to the Glenmalure valley and forest walks.