A boil water notice has been imposed on homes and businesses supplied by the Whitegate Regional Water Supply Scheme in east Cork following an issue with the filtration process at the plant.
The notice will affect some 9,500 people in areas including Whitegate, Aghada, Rostellan, Saleen, Cloyne, Ballycotton, Churchtown, Shanagarry and Ballinacurra.
In a statement issued this afternoon, Irish Water said it was working with Cork County Council to resolve the situation as quickly as possible.
The water supply board acknowledged the "significant inconvenience the boil water notice will cause" and it apologised to consumers.
It also acknowledged that there have been previous problems with the supply.
In early 2016, homes and businesses in the area supplied by the Dower plant were without drinking water for a number of weeks because of high turbidity levels.
At the time new micro filters were installed in a bid to safeguard future water quality.
People are being advised to boil all water for drinking, food preparation, brushing of teeth and making of ice, until further notice.