Irish Water is being prosecuted by the Environmental Protection Agency (EPA) for failing to comply with an order to upgrade a drinking water treatment plant.
The utility has been summonsed to appear in court after it failed to upgrade the Carraroe treatment plant in Galway by last November, and faces a fine of up to €5,000.
It comes after the EPA revealed that more than 803,000 people are drawing their drinking water from 119 plants considered to be at risk of failing to meet quality standards. This is up by 21,000 people since the end of 2015.
The plants, which are across 21 counties, are failing to remove dangerous bugs including cryptosporidium, are failing to meet standards to remove cancer-causing THMs, have excessive levels of pesticides or aluminium in the water or require operational improvements.
The EPA said there were "underlying problems" with the drinking water network, some of which were flagged as far back as 2008.
It would "escalate" enforcement action where Irish Water was not considered to be making efforts to comply with legally-binding directions, but it stressed that the water produced was meeting quality standards, but was produced by plants in need of major works.
"We're not saying they're breaching the standards, but that they're at risk of non- compliance," a spokesman said.
Details of the affected plants are contained in the Remedial Action List (RAL) for the first quarter of this year, which is published today.
■ Almost 40,000 people served by 11 plants in Cork, Galway, Roscommon and Tipperary are subject to boil water notices or restrictions.
■ Another 37 plants serving 130,000 people are failing to meet THM standards, and another 85,000 served by 17 plants are at risk of cryptosporidium.
■ Improvements are needed at another 50 plants, serving more than 540,000 people.
■ At four plants, serving 5,000 people, measures are in place to deal with cryptosporidium but are subject to verification they are working properly.
The RAL also shows that the EPA has issued 20 directions to Irish Water to complete works. Irish Water plans to abandon 28 plants or supplies, and connect the areas with another facility, and upgrade or build another 63 plants.
Some 90 of the 119 affected supplies will be dealt with by the end of 2018.
Other schemes, such as the €200m upgrade to the Vartry Reservoir, will not be completed until 2020 but will address concerns for more than 200,000 people.
Irish Water said that when it took control of the network in January 2014, some 140 plants were on the RAL and 900,000 people affected.
Since then, it had completed works at 57 plants, but another 36 had been added to the list. Some €2bn would be spent improving drinking water to 2021, including €327m addressing THMs.
A total of 29 plants would be removed from the RAL by the end of this year.
"By adopting a single national best practice approach to assessing and managing drinking water supplies, Irish Water has shown beyond any doubt that drinking water quality across the country has been seriously compromised by a systematic failure in how water services have been planned, delivered and funded over several decades," Mark Macaulay, water supply strategy lead at Irish Water said.
"Irish Water's work in the past two years has indicated serious compliance challenges ahead for hundreds of drinking water supplies."
He added there would be no schemes on the RAL by 2021.