People Before Profit TD Mick Barry has told the Dáil that there is a "water crisis" in parts of Cork city and the situation was a "fiasco".
He said household water in some homes is dirty and discoloured, and emerging as brown and orange.
The Cork North Central TD said the problem was affecting thousands of homes and had gone on for 18 months.
He said the problem began when a contractor for Uisce Éireann added too much caustic soda to the supply in order to soften hard water.
This washed away decades of sediment which went into the water supply, he said.
Some households are buying bottled water at a cost of €20 per week or €1,000 per year, Deputy Barry added.
Minister for the Environment Eamon Ryan said the Environment Protection Agency (EPA) is investigating and said the situation must be resolved.
Uisce Éireann must be upfront and transparent and must fix the problem, Mr Ryan added.
There should be no part of the country where people feel they have to buy bottled water, he said.
