A boil water notice has been issued for people living in parts of counties Dublin, Kildare and Meath.
The notice affects approximately 600,000 people supplied by the Leixlip Water Treatment Plant.
It covers parts of Fingal, areas in Dublin City Council, parts of South Dublin County Council, parts of Co Kildare and Dunboyne in Co Meath.
Irish Water says the areas affected include: Artane, Ashtown, Balbriggan, Baldoyle, Ballyboghill, Celbridge, Clonee, Clonsilla, Coolock, Coolquoy, Corduff, Darndale, Donabate, Dunboyne, Finglas, Garristown, Glasnevin, Howth, Kilbarrack, Kilclone, Killester, Kinsaley, Leixlip, Lusk, Malahide, Maynooth, Naul, Palmerstown, Poppintree, Ronanstown, Rush, Skerries, St Margaret's, Straffan, Sutton and Swords.
Irish Water said it expects the boil water notice will be in place for a couple of days.
Speaking on RTÉ's Drivetime Yvonne Harris, head of Customer Operations with Irish Water, said the water quality would be tested continuously and the notice would be lifted as soon as it is safe to do so.
She said Irish Water would be working with the Health Service Executive and the Environmental Protection Agency to do that.
According to Irish Water, the boil water notice was issued for the entire supply as a precautionary measure following issues with the treatment process at Leixlip, which may have compromised the disinfection process which makes the water unsafe to drink.
The issues have now been resolved but water is travelling through the system which is why the boil water notice is in place, according to Irish Water.
In a statement, Irish Water said it acknowledged the impact and inconvenience caused by the boil water notice to homes and businesses.
"We would like to reassure our customers that that Irish Water's drinking water and operational experts are working with our colleagues in Fingal County Council to resolve this issue as quickly as possible."
Further information on boil water notices can be found on the Irish Water website.
The website was temporarily unavailable this evening due to what appears to be a high volume of traffic.
The EPA said it will carry out an assessment of the performance of operations at the plant and will undertake an audit of the facility in the coming days.
In a statement it said the audit "will examine the treatment processes in place and the actions taken to address the issues which led to the boil water notice being put in place.
"The EPA will ensure that any recommendations of that audit will be implemented by Irish Water".
The notice advice is that water must be boiled for: drinking; drinks made with water; preparation of salads and similar foods, which are not cooked prior to eating; brushing of teeth and making of ice.