Most local authorities are losing 40% of their water supply through leaks, which is not expected to improve by the end of this year despite targets to address the problem.
Five councils expect to still lose most of their treated water to leaks.
The targets are in service plans that the local authorities agreed with Irish Water.
Figures have been released to RTÉ News under the Access to Information on the Environment regulations.
Irish Water has said one of the measures it will use to reduce leakages will include a reduction in water pressure in some parts of the network.
Each local authority has agreed a target with Irish Water to reduce the amount of water lost in its area.
In most counties the target is to reduce the leakage rates by two percentage points by the end of this year.
Nationally, the hope is to lower the leak rate from 49% to 47%.
In Cork city the target is to bring the percentage of water lost from 55% to 53%.
The other areas losing more than half their supply to leaks are counties Cork, Mayo, Kerry, Tipperary and Roscommon.
Roscommon at 62% has the worst problem.
It hopes to reduce that to 58% by the end of this year.
The figure for all local authorities in Dublin is 33%, with Dublin city losing 38% of supply through leaks.
Irish Water has said it will reduce the amount of water lost by fixing and replacing pipes and by reducing pressure in some parts of the network. They said fixing leaks in the water system is going to be a slow and systematic process that will take many years.
Speaking on RTÉ's Morning Ireland, Jerry Grant, Head of Assets at Irish Water, said pressure management will be a crucial part of this process.
Mr Grant denied that reducing pressure in certain areas will result in a reduction in flow.
He said he could reassure customers that Irish Water would maintain an appropriate pressure at the tap.
However, he said fixing the numerous leaks in the system will take time.
