A Do Not Consume notice has been put in place in the Threemilehouse area of Monaghan.
Irish Water and Monaghan County Council say almost 300 people on the Togan Public Water Supply are affected.
Elevated levels of manganese are being blamed for the notice.
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Irish Water says the water should not be consumed at all and boiling the water will not remove the manganese.
An alternative water supply is being provided from this morning at Threemilehouse crossroads and those using it are being asked to bring their own containers and boil the water before consumption.
