The Green Party leader Eamon Ryan wants the Government to put the brakes on Irish Water.
Mr Ryan is responding to a warning that up to €2bn is going to be overspent at the utility company.
Professor John Fitzgerald of the ESRI said the agency will pay more than 4,000 staff - when it only needs about 1,700.
The Green Party leader says the Government should admit they have got it wrong and start the set-up all over again.
Irish Water is obliged by law to pay for local authority employees around the country who work on the current system.
Those agreements will run up until 2026 and involve about 4,300 staff.
Professor John Fitzgerald of the ESRI said that is about 2,500 more than Irish Water actually needs and will see it overspend by €1bn to €2bn over the coming decade.
He said the scale of the waste involved in this structure makes all other problems associated with the establishment of the new utility "pale into insignificance".
It follows the news late last week that 29 staff recruited by Irish Water are earning over €100,000.
However, Professor Fitzgerald says setting up Irish Water will free an extra €500m in Budget 2015, which could be used for tax cuts or extra expenditure.