The country's largest ever boil water notice has been lifted.
600,000 people in Dublin, Meath and Kildare haven't been able to drink their tap water since a treatment issue on Monday.
Tests have shown the water is now safe to drink again.
A report found Irish Water failed to respond to a number of alarms, warning it there was a problem at its plant in Leixlip.
Yvonne Harris, Head of Customer Operations, said Irish Water "acknowledge and understand the impact" the move had.
"We sincerely regret the inconvenience", she added.
Irish Water says it has already made changes to catch failures at its Leixlip treatment plant faster.
