A Do Not Consume notice has been issued to consumers supplied by the Mountainlodge Group Water Scheme in Co Cavan.
It follows consultation between the HSE and Cavan County Council.
Boiling water will not render it safe to drink and water from the hot tap of kitchen or bathroom sinks is not safe to drink.
An alternative safe source of drinking water, such as bottled water, should be used for drinking, cooking food and food preparation, brushing teeth, and making ice.
Alternative water supplies will be provided and consumers are asked to contact Mountainlodge Group Water Scheme for further information.
People are also reminded to boil water from tankers before use for drinking or for food preparation.