Thousands of people are affected by a Do Not Consume water notice issued in Co Cavan.
Nearly 8,000 customers served by the Bailieborough Public Water Supply are being warned not to consume the water due to elevated levels of manganese.
The areas affected include Bailieborough, Killinkere, Virginia, Crossbane and Mullagh.
The notice by Irish Water follows consultation with the Health Service Executive and Cavan County Council.
Irish Water said boiling the water will not reduce manganese levels and is therefore not a suitable measure to make the water safe to consume.
In a statement, it said it is especially important that mains drinking water is not given to bottle-fed infants. The notice does not apply to other areas and water schemes in Co Cavan.
Manganese is found naturally in many lakes, rivers and groundwater sources. Water passing through soil and rock can dissolve minerals containing manganese. The levels leaving the water treatment plant exceed those permitted under the drinking water regulations, the statement adds.
Irish Water's Peter Gallagher said alternative sources of water are being provided for drinking and people can continue to use the water for flushing of toilets, bathing, laundry and washing utensils.