The results of tests carried out in Galway confirmed this evening that the city and county's water supply is contaminated by a resistant parasite which can cause illness with severe cramps and diarrhoea.
Evidence of the cryptosporidium organism was found in the Terryland city waterworks and Headford, Co Galway.
The Health Service Executive has advised over 90,000 living in the city and in an area of the county between Moycullen in Connemara and Tuam and Athenry to continue boiling tap water before drinking it or using it for food preparation.
The drinking water alert was highlighted after public health officials in the county noticed a steep rise in the number of people suffering from a stomach bug illness in recent weeks.
Lastest figures show that the number of people affected by the bug has risen to 60.