Irish Water has announced plans to extend the reach of the Lough Mask supply scheme in Co Mayo to parts of Galway and Roscommon.
Williamstown in Co Galway and Ballinlough in Co Roscommon will be added to the network as part of a €10m investment.
The development will entail over 20km of new pipelines, a new pumping station at Ballyhaunis and the construction of a new reservoir at Ballinlough.
Irish Water says the extension will enhance supply to over 5,000 customers and will lead to the end of an ongoing boil water notice for homes in the area of Co Roscommon in question.
It is the first time that parts of three counties will be supplied by a single supply scheme.
An existing treatment plant near Williamstown will be decommissioned when the new infrastructure is completed.
Residents there had to contend with a lengthy boil notice last year, after the presence of cryptosporodium was detected in the supply.
The works are expected to take one year to complete.