A fire in an electrical panel in Freemount water treatment plant will affect the water supply of approximately 3,500 people in the Freemount, Milford, Liscarroll, Dromina areas.
Irish Water said the fire broke out at approximately 3pm and that while no one was injured, the blaze meant critical equipment at the plant is not available to produce water for the Allow Regional scheme.
"Irish Water and Cork County Council are working with specialist contractors to assess the impact and the extent of works involved to safely restore normal production as soon as possible," Irish Water said.
"Storage in reservoirs is expected to be depleted by tomorrow morning.
"In order to minimise the extent of supply interruption, and while works are progressed to address the damage at the treatment plant, Irish Water have this evening commenced transporting water to the reservoirs serving the scheme and will expand these efforts tomorrow and over the coming days."