The company said that 472 plants posed a risk, some operating above capacity, and others have no alarms to warn of problems or they draw water from poor quality sources.

New quality guidelines mean water is now tested up to six times a year, and that Irish Water intends to shut down plants and issue boil-water notices if a problem emerges. This is unlike the system operated in the past, where boil-water notices were implemented after water was found to have failed to meet quality standards and posed a risk to human health.

“It is possible that more boil-water notices will have to be issued, even in places where no problem appears to exist today,” a spokeswoman said.

“We’re now finding problems which existed before, but were never picked up. Boil-water notices were traditionally issued on a reactive basis, but we’re looking for problems.

“We believe about half of the plants are not capable of dealing with all conditions for the raw water they are treating. If they don’t meet the standard, they have to be dealt with.”

The warning comes after boil-water notices were put in place for Carraroe in Galway and Whitegate in Cork earlier this month, which affected more than 16,000 households.

Irish Water also says that some plants built in recent years were not designed to treat ‘raw’ water.

Different water types require different methods of treatment to make them safe for human consumption.

A briefing note states that drinking water quality has been “seriously compromised” by a “systematic failure” in funding and delivering water services over several decades.

“Irish Water’s work in the past two years has indicated serious compliance challenges ahead for hundreds of drinking water supplies as a result of the first ever comprehensive evaluation of the performance of drinking water production plants,” it says.

It says that new plants were often developed based on “limited compliance monitoring”. This means the type of water being treated was not factored into the plant’s design.

“This explains why many recently upgraded water supply schemes are currently failing on health-related compliance parameters intermittently or in extreme cases regularly,” it adds.

“Treatment plants and networks were not upgraded or maintained because the money and resources were not available to do so. Many are therefore no longer fit for purpose or are in need of significant investment.”

Some 35,000 households across the network are currently affected by boil-water notices.

Some 472 plants fall into the ‘at risk’ category, from a total of 856.

Most of these plants are smaller and are located in rural areas, and serve around 240,000 of the utility’s 1.5 million customers, or 16pc.

Sources said some plants were overloaded, and operating at up to 120pc of design capacity.

“The water is compliant and safe to drink, but it’s like running your car at 7,000 RPM and not changing up a gear,” one said.

New systems require ‘raw’ water to be tested at least six times a year, across different seasons, to determine how its properties change.

Irish Water has submitted a detailed €3.5bn investment plan to the Commission for Energy Regulation, setting out upgrades of 340 drinking and wastewater plants needed over the next five years.

It said around half of the country’s drinking water plants will be amalgamated or shut down over the coming years, with some networks joined to larger plants.

Galway

Loughrea – needs ultra-violet (UV) treatment, which kills bacteria, to remove the risk of cryptosporidium.

Kilconnell – needs to protect the source from livestock, and UV treatment.

Carraroe – needs specialist treatment for cryptosporidium and address the risk of THMS, a group of organic chemicals often present in drinking water and formed when chlorine reacts with raw water.

Wicklow

Aughrim – the plant was upgraded but filters designed to remove organic material from raw water proved unviable, resulting in THM issues. The new filters are not in use as they are ineffective.

Laois

Ballyroan – required a new UV system.

Cork

Whitegate – the UV treatment was not suitable to deal with the type of raw water, A new filtration and UV unit are required. 10,000 people are on a boil-water notice.

Mayo

Mask – the newly constructed phase in the last decade was unable to meet its quality standard. It is now undergoing a major upgrade.

Meath

Drumconrath – A previous plant upgrade is ineffective at removing organic material and consequently legacy THM issues must be addressed.