Drumharlow Lough, also known as Lough Eidin or Lough Drumharlow, is a lake in County Roscommon, Ireland, situated near Carrick-on-Shannon. It covers a surface area of approximately 2.8 km² (1.1 sq mi) and sits at an elevation of 43 m (141 ft). The lake lies on the Boyle River, just above its confluence with the River Shannon, placing it within the broader Shannon waterway system that connects it to an extensive network of navigable waters.

The lake contains an island called Inishatirra and features extensive callows — low-lying, seasonally flooded grasslands — along its shoreline. This shoreline habitat is proposed as a Natural Heritage Area, reflecting its ecological significance. The callows support a flock of Greenland white-fronted geese (Anser albifrons flavirostris), a species of conservation concern, making the lake of particular interest to wildlife observers. Fish species present include roach, bream, brown trout, and rudd.

The lake's position on the Boyle River, immediately upstream of the Shannon junction, means paddlers can incorporate it into longer river touring routes connecting the Boyle River system with the Shannon. The nearby town of Carrick-on-Shannon provides the closest settlement and likely the most practical base for access and logistics. A regatta was held on the lake in 1895, though it was not repeated, with accounts noting the lake was considered too small for the event — a detail that gives some sense of the lake's modest scale relative to other loughs in the region.

The proposed Natural Heritage Area designation of the shoreline may carry implications for paddlers regarding access or conduct near sensitive habitats, and it would be worth checking with local authorities or conservation bodies for any specific restrictions before visiting.