A recent presentation to Monaghan County Councillors on the water quality of Lough Muckno highlighted how poor it was.
While it won't impact on the area's drinking water, people are being advised not to bath or swim in the lake.
A plan is currently being devised by Monaghan Co Council under the Local Authorities Water Programme.
The focus will be on upgrading the water within the catchment area.
Cllr Aidan Campbell said efforts were already underway to deal with the problems that have emerged.
But he warned that it's going to take a number of years for improvements to be made.