1,200 customers had been affected by the fault.
A boil water notice in south Offaly has been lifted with immediate effect.
The advisory had been in place for 1,200 customers supplied by the Dunkerrin Public Water Scheme since September 22.
It followed a mechanical failure at the water treatment plant which affected the disinfection process.
However, Irish Water says the supply has now been deemed safe to drink again after an audit by the Environmental Protection Agency.