600,000 people in Dublin and surrounding counties are being told to boil their drinking water.
The order follows a mechanical error at a water treatment plant in Leixlip, Co Kildare.
The boil water notice applies to parts of Fingal, areas in Dublin City Council, parts of South Dublin County Council, parts of Kildare and Dunboyne in Meath, and will remain in place over the coming days.
Yvonne Harris, from Irish Water, said that the mechanical error has been fixed but the untreated water remains in the system.
We estimate that 20% of the water in the pipes at the moment is not treated as well as it should be, although 80% is.
"We would ask our customers to boil water for drinking, for preparing food, for brushing teeth, for any consumption purposes.
"The water is safe to bathe in, to shower and wash in."