Water supplied by the plant at the centre of the country's biggest-ever boil water notice is still not being treated thoroughly enough to safeguard against contamination.
A report by the Environmental Protection Agency (EPA) on last week's crisis at the Leixlip Water Treatment Plant says Irish Water should employ extra disinfection techniques as an added protection.
It says the problems that led to more than 600,000 people in Dublin, Kildare and Meath being unable to drink from their taps for three days for fear of contamination by cryptosporidium or giardia parasites were down to "unacceptable" failures by Irish Water.
It found a blockage at the plant was caused by bits of the lining of 25-year-old tanks coming loose and making their way into the equipment.
That blockage put pressure on the system that caused a relief valve to blow off, but pumps that add a product called alum to the water to capture dirt and contaminants kept working at full power.
As a result, half of the alum ended up dumped in overflow tanks and only half reached the raw water it was meant to be purifying.
Alarms sounded to notify staff at the plant that there was a problem - but they were not responded to, and the half-treated water continued to flow into the public drinking supplies for 11 hours.
A similar incident occurred last March and the EPA told Irish Water then to install an automatic plant shutdown if alarms were not responded to after 15 minutes, but this did not happen.
"The failure of Irish Water and Fingal County Council [which operates the plant on Irish Water's behalf] to implement the recommendations of the EPA's previous audit of Leixlip water treatment plan in March 2019 is unacceptable," the report says.
"The alum dosing incident which occurred on October 21, 2019, and the unacceptable failure to respond to multiple process alarms presented a significant risk to the safety of the water supply.
"Irish Water and Fingal County Council must ensure there is a system in place to respond adequately and appropriately... and cannot rely on human intervention alone."
The report notes that there were already 10 detections of giardia and one of cryptosporidium at the plant this year before last week's incident, and says: "The level of treatment at Leixlip water treatment plant is currently not proportionate to the risk posed by the River Liffey source water."
It recommends Irish Water consider installing an ultraviolet disinfection system as an extra precaution.
It also says that the Leixlip plant should be connected to Irish Water's national control centre "without delay" to allow centralised oversight of its operation.
It notes that other recommendations for improvement works at the plant that followed the March incident had been delayed to next year or 2021.
Irish Water has until November 30 to present a plan of action to the EPA in response to the audit.
In a statement, the company said the events at the plant "fell well below our standards" and it sincerely regretted the inconvenience caused to homes and businesses.
"Irish Water and Fingal County Council are currently reviewing the findings of the EPA audit of the Leixlip Water Treatment Plant.
"Following that review, we will outline the plan for the implementation of the EPA's recommendation, including details on the actions we have already taken and those in progress," it said.
Irish Water management and representatives of Fingal County Council are due before the Oireachtas Housing Committee to answer questions about the problems at the plant next Tuesday.
Housing Minister Eoghan Murphy has also requested a separate report from the EPA on the incident.