Longford residents have been hit with a Boil Water Notice with 2,500 people affected.
Irish Water said a mechanical issue at the Lough Kinale water treatment plant resulted in the treatment process being compromised.
Customers who are served by the Granard Public Water Supply are advised to boil their tap water before drinking it.
The areas affected include Granard, Abbeylara, Aghnagarron, Ballynacross, Granardkill, Cartron, Robinstown, Higginstown, Carrickduff, Drumincrehir, Culleenmore, Corbaun, Killasona, Lisryan, Cloonshannagh, Ballagh Gowra and Froghan.
Irish Water say they are working with Longford County Council to resolve the issue.
The company said it will continue to "liaise and consult with the Health Service Executive with a view to lifting the Boil Water Notice as soon as practicable".
A spokesperson said: "Irish Water acknowledges the impact and inconvenience caused by the imposition of a Boil Water Notice to homes and businesses.
"We wish to thank the community for their patience and cooperation while we work to resolve this issue. Irish Water’s priority is the provision of safe, clean drinking water and safeguarding that water supply for the future is a vital focus.
"Irish Water is contacting registered vulnerable customers who are affected by this Boil Water Notice to advise them.
"Should customers have any queries regarding this Boil Water Notice, they should contact Irish Water directly on our customer care helpline."
It comes as more than 600,000 people remain under a separate Boil Water Notice in parts of Dublin, Kildare and Meath.
The notice - which has been in place since Monday evening - is the second that's been issued for customers served by the Leixlip Water Treatment Plant in the past two weeks.
A test carried out on the water quality on Saturday returned satisfactory, Dublin Live reports.
However, Irish Water say the Environmental Protection Agency and the Health Service Executive want further tests before the boil water notice can be lifted.
The water quality tests will continue until Tuesday.
