Up to 3,000 households and businesses are affected by the rupture of a water-main in Leixlip.
The incident, on Station Road, emerged at around 5am this morning, and it could be 8pm this evening before repairs are completed.
It could take up to three hours, thereafter, for the system to refil.
A traffic management system will be in place for the duration of these repair works.
This was a pipe used to transfer water to Ballygoran Reservoir, which also supplies Celbridge.
Homes and businesses there are affected, as well.
Regional Operations Manager at Irish Water, John O'Donoghue