Irish Water was warned five-and-a-half years ago about the problems that led to the country's biggest-ever drinking water contamination scare.
An assessment of works needed at the Leixlip Water Treatment Plant in July 2014 reported that filters were failing and may not be removing bacteria and parasites, including cryptosporidium.
It was among a number of assessments that detailed serious deterioration and neglect at the plant.
It was found to be operating with out-dated technology, corroded parts and unreliable back-up systems from an overcrowded, leaking premises described as a fire hazard with likely asbestos contamination.
All the assessments stressed the national strategic importance of the plant, which is the country's second largest and supplies 660,000 people in Dublin, Kildare and Meath, including homes, businesses and major employers.
One assessment stated: "There has been a noted deterioration in the filtrate quality from all 15 rapid sand filters."
The filters were "essential" to removing particles and impurities it said, warning: "With continued deterioration of these sand filters, the risks of cryptosporidium breakthrough and contamination of the treated water will increase further."
Yet filter replacement began only last July and is expected to take until this summer to complete.
In the meantime, those 660,000 people were placed on boil water notices last October and again in November because of fears of parasitic contamination, causing major disruption to households and business.
Irish Water said the filters could not all be replaced at once as the water supply to the region is so stretched that the plant had to keep operating at full or near to full capacity at all times.
That is still the case although some production has been transferred to another plant to try to speed up the work.
Irish Water has warned there is no guarantee that conditions will not deteriorate and said further boil water notices might have to be declared before the replacement work was completed.
The company took charge of the country's water services only in 2014 and the scale of the task of bringing the Leixlip plant up to standard is clear from the assessments carried out between then and 2016.
The administration and control building (ACB), which was built in the 1960s, had never undergone refurbishment or upgrade.
"There is evidence of cracking in the structure and possibly some subsidence. There is asbestos in the adjacent redundant old pump room and therefore possibly also in the ACB," one report said.
"Leaks have appeared in the external walls and the roof. The accommodation is overcrowded for the staff."
It added that the position of the stairs in the centre of the crowded building was a fire hazard. The building was also "unsightly", it said.
Another report said the many electrical control panels throughout the plant were fitted with automatic fire extinguishers that were more than 20 years old and had never been serviced.
"The entire system is ineffective, with many parts perished," it said, warning that a fire would completely destroy the electrics and knock the plant out of production.
"Not only does the current situation represent a serious fire risk, but it may also compromise Irish Water's fire insurance policy."
Irish Water said the system had since been replaced and that improvement works had been carried out on the ACB.
Sludge presses at the plant had not been serviced for years and were operating below capacity so that they were not removing sufficient water from the sludge sent to landfill.
As a result, the sludge was heavier than it should be and an extra €130,000 a year was being spent on transporting it.
The general control system was 20 years old and parts were hard to source.
"At the time of installation in 1995, they would have been considered leading-edge technology" but they were now "legacy equipment", a report stated.
A unit that controlled valves in the event of a power failure was itself prone to failure.
"Without this unit to control the valves in the event of a power failure, raw water will continue to enter the plant without being treated," a report warned. "It's critical that this unit be replaced as soon as possible."
A pump and stand-by pump at the sludge plant were "old and not reliable".
"There is a risk of sludge being discharged into the river where the EPA sample weekly," that report warned.
A number of 'clarifiers' in one part of the plant suffered corrosion, collapse and deformity.
There were also problems with the equipment that mixed the water and coagulant - a substance used to extract solids from the water - so extra chemicals were being used to boost the effectiveness of the process, which was not considered best practice.
A disinfection system was also prone to failures and there was no back-up system, although one has since been installed.
Irish Water has not released full details of the works required, citing commercial sensitivities and potential difficulties in negotiations with staff unions, contractors and local authorities.
But it stressed that numerous works had been carried out in response to the reports and said further works were scheduled for 2020 and 2021.
It said this now included ultra-violet disinfection which the Environmental Protection Agency asked Irish Water to consider putting in place as an added safeguard for public health following the October and November incidents.