Approximately 600,000 people are affected by the notice.
A boil water notice has been issued for parts of Dublin, Meath, and Kildare.
Irish Water has issued the warning for homes and businesses in the vicinity of the Leixlip Water Treatment Plant. The entire water supply has been affected.
Issues with the water treatment process led to the notice last night.
Irish Water have said that these issues have since been resolved, but the boil water notice will remain in place for a few days as water continues to travel through the system.
"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," they said.
In this instance, water must be boiled and cooled before drinking, brushing teeth, gargling, preparation of food, making ice, and drinking filtered water.
According to the Irish Water website, a boiled water notice is usually issued when routine testing of the drinking water supply has shown the presence of harmful bacteria (such as E. coli), or pathogens such as Cryptosporidium.
A notice can also be issued "where there is a risk of contamination but where test results are yet to be confirmed."
"An example might include where the disinfection system has failed, or where a pollution event has occurred in the vicinity of the source water."
The affected areas are:
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.
