Kerry County Council has issued an alert to more than 4,000 homes in the north of the county after a water source supplying some villages in a public scheme was found to be contaminated.
Mr Vincent Horgan, the council's area engineer, described the contaminant as "bacterial" but said a higher than usual level of ammonia was also found in the normally pristine ground water source. The amount of ammonia found was not above accepted levels, he said.
Historically, the Ballyheigue and north Ardfert public water supply source had been excellent, Mr Horgan said.
The problem was discovered during routine testing on Monday.
Householders in Ballyheigue, Kerry Head, Kilmoyley, Lerrig, Causeway, Clanmaurice and the area north of Ardfert village were urged to boil their drinking and cooking water until the alert was lifted.
The water should be boiled for "a minimum of 10 minutes", Mr Horgan said.
The presence of the higher-than-usual levels of ammonia added to the belief that some form of pollution had occurred. The council was trying to trace the source, Mr Horgan said. Heavy rainfall at the weekend may have contributed.
Further testing was taking place and the council hoped to be able to lift the alert very soon.