| Glencar Lough | |
| Pushpin Map: | Island of Ireland |
| Pushpin Map Alt: | Glencar Lough location in Ireland |
| Pushpin Label Position: | none |
| Pushpin Map Caption: | Location in Ireland |
| Location: | County Leitrim, County Sligo |
| Lake Type: | Glacial lake |
| Inflow: | Glencar Waterfall, Diffreen River |
| Outflow: | Drumcliff River |
| Catchment: | 41.22km2 |
| Basin Countries: | Ireland |
| Length: | 2.5km (01.6miles) |
| Width: | 0.6km (00.4miles) |
| Area: | 1.15km2 |
| Elevation: | 28m (92feet) |
Glencar Lough, locally known as Glencar Lake, is a freshwater lake in the northwest of Ireland. It covers an area of 1.15km2 and lies mostly in County Leitrim with a smaller part in County Sligo. Glencar Waterfall is located near the lake's north shore on the Leitrim side.
Glencar Lough lies in the Glencar Valley, between the Dartry Mountains to the north and the mountain range including Cope's Mountain to the south. The lake is located about 10km (10miles) northeast of Sligo and about 150NaN0 west of Manorhamilton. It is 2.5km (01.6miles) long from west to east and 0.6km (00.4miles) wide. The lake has two crannogs (artificial islands): one at the western end near the Drumcliff River outlet and the other at the eastern end near the Diffreen River.
Glencar Lough is primarily fed by Glencar Waterfall, on the lake's northern shore, and by the Diffreen River, entering at the lake's eastern end. Sruth in Aghaidh an Aird also flows out to the northern shore, just west of the Glencar Waterfall outflow. The lake drains west into the Drumcliff River, which in turn flows into Sligo Bay. Lake depth is greatest near the southern shore with a shallower shelf at the northern shore.
Fish present in Glencar Lough include salmon and brown trout. Bird life includes tufted duck, pochard and goldeneye. These are migratory species which winter at the lake.
The water quality was reported to be excellent with an oligotrophic rating. The ecology of Glencar Lough, and other Irish waterways, remain threatened by curly waterweed, zebra mussel, and freshwater clam invasive species.
Historically Glencar Valley was known as Glenn-Dallain and was part of the Kingdom of Breifne. The lake and its crannogs, then occupied, are mentioned in the Annals of the Four Masters, specifically the eastern crannog where "the sons of Donough O'Rourke, i.e. Donnell and Ferganainm, made an attack upon the crannog, and privately set fire to the town".
Barite was mined at Glencarbury in the Dartry Mountains above the lake between 1894 and 1979. A cable ropeway from the mine area to the lake shore was constructed in 1942. By this means the extracted barite was taken down for onward road transport.
The Glencar Water Company has its bottling plant located near the lake's northern shore. The company, established in 2008, sells water drawn from an onsite spring emanating from within the nearby Dartry Mountains.