At least 52 people fell ill after recent incidents led to unsafe water entering public supplies, officials have said.
One incident happened last month at a plant in Wexford which serves Gorey, while a second failure occurred at Ballymore Eustace which serves large parts of Dublin.
The EPA has sharply criticised the "abject failure" that led to delays in notifying both themselves and the HSE about the issues.
In the Gorey water treatment plant incident, a power failure and a chlorine pump failure resulted in water leaving the plant and entering the public supply "without the appropriate level of disinfection" for around five days (between August 19th and 24th).
The EPA and HSE weren't told about the incident until August 26th, which the EPA says may have prevented a timely public health intervention.
There's said to have been 52 confirmed cases of illness associated with a 'public health outbreak' in Gorey, including several hospitalisations.
VTEC (a dangerous form of E. coli) bacteria is said to have been involved in some of those illnesses.
Meanwhile, the other incident saw unsafe drinking water leaving the Ballymore Eustace plant - the largest water treatment plant in the country - for up to 10 hours on 20th -21st August, "due to the loss of the Cryptosporidium treatment barrier compounded by inadequate disinfection".
Officials in the HSE and EPA weren't told about that incident until September 1st, over a week later - with the EPA again saying this led to a delayed response to the incident.
EPA investigations at two drinking water treatment plants revealed abject failure of managerial oversight, operational control & responsiveness by @IrishWater & local authorities in terms of their respective roles to deliver safe and secure drinking water https://t.co/UOanVhW4gx pic.twitter.com/LHCz4tql4f
— EPA Ireland (@EPAIreland) September 17, 2021
In a statement, the EPA said there had been an "abject failure of managerial oversight, operational control and responsiveness" by both Irish Water and local authorities.
It says there were "unacceptable delays in reporting", meaning up to 900,000 people weren't issued with boil water notices.
Local Government Minister Darragh O'Brien says the incidents and delays in reporting have left him "furious".
He said: "It's completely and utterly unacceptable."
He's due to meet with Irish Water and Dublin and Wexford local authorities this morning, and he has called for an immediate report to ensure water supplies are safeguarded in future.