A boil water notice that has been in place in Wicklow and south county Dublin since Monday has been lifted.
Just one area, within the Woodstock Business Park in Kilcoole, remains subject to the notice.
The notice had affected some 65,000 people.
It was in place as a precautionary measure following a mechanical failure at the Vartry Water Treatment Plant.
The failure affected chlorine equipment at the facility.
Irish Water has apologised for the inconvenience caused to residents by the notice over the past four days.
Irish Water just last week turned the sod on the construction of a new pipeline from the Vartry plant to Callowhill.
The €29m project will see a 150-year-old tunnel being replaced.
The utility was granted planning permission in February 2017. Works are due to get underway later this year and are due to take two years to compete.