Monaghan County Council are advising people not to swim in Emy Lough until further notice.
It comes following a Poor Water Quality test result at the lake earlier this week.
Bathers are being advised not to swim in the lake due to increased levels of bacteria found in a sample on Tuesday. To reduce the risk of illness users should avoid swallowing or splashing water, avoid swimming with an open cut or wound, and avoid swimming if pregnant or with a weakened immune system. Monaghan County Council have arranged for further samples of the water to be taken in the meantime