UP to 1.3 million people are drinking mains tap water which is not safe because the plants treating it are old and need to be replaced.
he Irish Independent has learned that 290 treatment plants across the State must be upgraded because they are out of date and not making the water safe for drinking.
Dangerous levels of bugs, including e-coli and cryptosporidium, are being detected in supposedly treated water, while chemicals are also being found.
A report to the Government from the Environmental Protection Agency (EPA) repeatedly warns that minimum safe standards are not being met in water treatment plants throughout the nation.
The EPA said that treatment plants serving small populations, as well as major towns and cities, were beset with problems.
The reason is because of decades of under-spending in new water treatment plants.
Tens of thousands of new homes were built during the boom, but treatment plants were not expanded and upgraded to meet the demand.
While many people who drink untreated water can become ill, most would not require medical attention.
But in Galway in 2007, 40 people had to be hospitalised after the city's supply became contaminated with cryptosporidium.
More than 70,000 people were forced to buy bottled supplies for five months, and the EPA later revealed that testing for cryptosporidium failed to meet the required standards, despite the water being drawn from the River Corrib which was a high-risk supply.
During the crisis, 40 people became seriously ill and had to be hospitalised. Experts said that up to 2,000 people may have been affected, but did not require medical treatment.
Even now, three years after the country's worst public water contamination crisis, many people in Galway are still choosing not to drink their tap water.
Former Galway mayor, councillor Catherine Connolly, was among those who came down with diarrhoea, abdominal cramps and a general unwell feeling.
And despite the rigorous testing the city's water supply now undergoes, she is among the many in Galway who choose to drink bottled water.
"I know I am a public representative, but I have never recovered my confidence in the public water supply," she said.
The 290 plants in question serve almost 1.3 million people, or one in three of the population, and are located in every county. The Department of the Environment admitted that many of the plants needed to be upgraded, but that a lot of the work was minor.
Environment Minister John Gormley is expected to be allocated €500m next year to complete works on treatment plants across the country.
"Some of them can be resolved pretty quickly but some require bigger solutions," a spokesman said.
"These are all public supplies that need investment or need to be brought up to a certain standard. For a lot of these schemes money isn't an issue because they're fairly low-cost solutions. Some €500m is being spent under the water services investment programme."
Notices that advise boiling water have been put in place in Kerry, Galway, Cork, Roscommon, Limerick, Longford, Tipperary and Clare so far this year after safety concerns were raised.
While most supplies across the country are safe, treatment plants serving cities including Cork and Waterford need to be upgraded. A report from the EPA shows that, in Waterford, the plant serving more than 40,000 people was not meeting EU standards to remove e-coli from the supply. It was upgraded, and experts are monitoring the source to ensure the action plan worked.
In Cork, the EPA says that treatment and management issues need to be addressed, including improvements in how the water is treated. The plant serves 123,000 people and was flooded last year.
There are more than 900 public water supplies, serving 87.5pc of the population. The rest are supplied by group water schemes (8.2pc), small private supplies (0.4pc) and single house private wells (3.9pc).
Responsibility for the water quality rests with the manager/operator of the supply.
In 2008, the last year for which figures are available, e-coli was found in 0.55pc of treatment plants. In England and Wales, it was in 0.02pc.