Glendalough Upper Lake, Glacial lake in County Wicklow, Ireland

Glendalough Upper Lake is a ribbon lake in the Wicklow Mountains, west of the Glendalough monastic settlement. It was once joined to the Lower Lake, but sediment from an inflow divided the original body of water in two. Near the southern shore stands the small rectangular church of Temple-na-Skellig, reachable only by boat and a flight of steps from the landing stage.

The valley and its lakes were shaped by glaciers during the last Ice Age, which cut deep into the rock and left behind a hollow that filled with water as the ice retreated. The two lakes formed separately because glacial debris piled up between them, creating a natural divide that is still visible today.

The cave high in the cliff above the southern shore is called St. Kevin's Bed, reputedly a retreat for St. Kevin and later for St. Laurence O'Toole. It can be picked out from the lakeside, but it sits well above the water and is not reached on foot.

The Wicklow Way, the long-distance trail from Rathfarnham to Clonegal, passes close to the eastern tip of the lake. The southern shore holds the boat-only church of Temple-na-Skellig, reached by a flight of steps from a landing stage. Part of the shore is marsh with horsetail, bottle sedge and common reed, where dragonflies are easily seen.

A small medieval church called Temple-na-Skellig stands on the southern shore and can only be reached by boat, followed by a climb up stone steps from a landing point. Because of this, most visitors to the valley have never seen it up close, even after multiple trips.