The ESRI has said the allocations for free water for children may not be correct.
Last week the Commission for Energy Regulation announced the allocation was to be 21,000 litres per year for each child.
The Department of the Environment's original figure announced last May was 38,000 litres.
In today's Sunday Times, associate research professor from the ESRI Edgar Morgenroth is quoted as saying the 21,000 litre calculation may be wrong.
Morgenroth said he cannot stand over Irish Water's data as the sample of metered households used for the research may be unrepresentative.
He added analysis depends on the type of estates where the meters are installed, the time of day the survey was carried out and may not have factored in seasonal variations.
He has recommended that Irish Water and the Commission for Energy Regulation "urgently revisit" assumptions on water consumption.