Thousands of people in County Cavan have been told not to drink their tap water.
It comes after high levels of the naturally occurring mineral manganese were detected in the Bailieborough public water supply.
7,785 homes and businesses are affected in Bailieborough and surrounding areas.
People are being warned that boiling the water won't make it safe and it is vital not to use it to feed babies.
Anthony Skeffington, from Irish Water, said the utility is trying to find out why manganese levels have increased.
“Over the past number of weeks there has been an unprecedented increase in the level of manganese in the raw water,” he said.
“Our plant there, the treatment system, has been unable to cope with that increase and therefore we are unable to supply compliant water from the plant to the general public.”
Manganese is an essential mineral; however, it becomes toxic when concentrations in the body become too high.