It'll affect those on the Dunkerin public water scheme.
Irish Water is issuing a boil water notice for 1,200 of it's Offaly customers.
It'll apply to all those who are served by the Dunkerin public water scheme after a mechanical failure at the water treatment plant led to the disinfection process being compromised.
Those in Dunkerin, Lisryan and the surrounding areas will be impacted however those in Moneygall village are connected to a separate supply.
Offaly County Council say experts are working to resolve the issues as soon as possible.
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