A boil water notice for some 600,000 people is set to be in place until at least Tuesday next week.
The alert remains in place for customers supplied by Leixlip water treatment plant.
It affects areas of Dublin, Kildare and Meath.
The boil water notice was re-issued on Monday for customers with recent bad weather being blamed.
It said the heavy rains led to unacceptable levels of turbidity (cloudy water due to suspended particles levels) ion the source water.
Irish Water says sampling at the plant is continuing.
The Environmental Protection Agency (EPA) conducted an audit of the plant on Friday.
Irish Water and Fingal County Council were on site to support the audit - the Health Service Executive (HSE) was also present.
The test results of three water samples taken throughout the week were found to be 'satisfactory' and these results formed part of the audit process.
The HSE determined that three satisfactory consecutive samples are required after Wednesday before the boil water notice can be lifted.
Irish Water says work will continue over the weekend to complete the testing - but the final test results will not be available until Tuesday.
Following a site meeting the HSE, EPA, Fingal County Council and Irish Water discussed the audit and the water sampling results.
The HSE was satisfied with the information and risk assessment provided by Irish Water to support the ultimate lifting of the boil water notice.
The HSE, EPA, Fingal County Council will continue to liaise on the Leixlip water treatment plant.
Niall Gleeson is managing director of Irish Water.
"Irish Water acknowledge and understand the impact of this boil water notice on the 600,000 people affected and we sincerely regret the inconvenience."
"We endeavoured to keep the public up to date at every stage and we are grateful to the media, elected representatives and members of the public who shared the information on social media and who supported family, friends and neighbours."
"We are grateful to our partners in Fingal County Council who worked with us to provide all of the necessary information to the HSE and EPA to facilitate the lifting of this boil water notice.
"Since early this morning Fingal County Council flushed the network in key areas to remove the remaining at risk water from the network."
Those who have queries regarding this notice can contact Irish Water directly on our customer care helpline on 1850-278-278.