The works will lead to a road closure.
Residents in one Offaly village can look forward to a more reliable water supply.
Irish Water is continuing its upgrade of the county's network, with works set to begin in Pullough.
Crews will replace four kilometres of mains prone to leaking and are expected to be finished on site by the end of July.
The works will run from the junction of the Pullough Road and Derrycooly to Rahan Road before continuing northbound towards Grand Canal.
On reaching Pullough’s Community Shop, the section of mains replacement will loop back towards the Pullough Junction at Heathfield and continue southbound along the Back Road to the first junction at Island Lower.
The works will result in a road closure.
The closure will take place from the junction of the R357 and Rahan Road, heading towards Rahan, before continuing Northbound to Pullough Village.
It will continue across the Grand Canal, via Pollagh's Community Centre, finishing at the Ballycumber road opposite St. Manchan's Church.
Northbound traffic to Pullough village will be diverted westbound along the R357 to Cloghan Village before taking the N62 to Ferbane village and continuing eastbound along the Ballycumber Road (R436) to St. Manchan's Church.
Irish Water says local residents have been notified of the road closure and the diversion.
Joe Carroll, Leakage Reduction Programme Regional Lead with Irish Water, says;
"Working in partnership with Offaly County Council, we have prioritised leakage reduction works to those areas which need it most. Replacing this old water main in Pollagh will reduce the number of bursts and outages.
"The works will safeguard the supply in Offaly now and into the future and support growth and development in the area.
"We understand that this type of work can be inconvenient but work crews will make every effort to minimise disruptions. Access for local and emergency traffic will always be maintained."