A Do Not Consume notice has been put in place on the Dernakesh Group Water Scheme in Cavan.
It's the third such notice put in place in the Northern Sound region in recent days.
Cavan County Council, in consultation with the HSE, has issued the notice after elevated levels of manganese were found in the water supply.
It says testing detected the elevated samples and as a result the water is "considered to be significantly compromised and an immediate risk to your health".
It's warned that boiling the water will not render it safe to drink and alternative water sources should be used.
It follows two other such notices put in place.
Over 400 customers in the area are affected and a water tanker has been place on Main Street in the village.
Another Do Not Consume notice was issued for the Killsherdany Group Water Scheme in Cavan on Friday, again because of elevated levels of manganese.