Almost 6,000 customers are affected.
A boil water notice has been issued for an area in Offaly.
Issues with the Roscrea Regional Public Water Supply and parts of the Shinrone/Brosna Public Water Supply are impacting customers in Cooleeshill, Killavilla, Whitepark, Sheehane, Cloonagh, Shinrone Village, Brosna, Dungar and surrounding areas.
In Tipperary, Roscrea town and surrounds: Glenbeha, Camlin, Scart, Gortmulin, Thesheehys, Derrymore, Racket Hall, Monaincha, Pintown, Irby, Borris and surrounding areas are affected.
The treatment process at the plant has been impacted by poor water quality in the Little Brosna River, with higher levels of ammonia.
Uisce Eireann have warned that all customers affected must boil water before use until further notice.
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