A boil water notice issued earlier this month remains in place for parts of Co Longford.
Irish Water and Longford County Council issued the notice on 3 November after consultations with the HSE.
Irish Water said it is not possible to say how long the boil water notice will last.
Approximately 17,500 customers are affected by a deterioration in raw water quality which "compromised the cryptosporidium at the treatment plant".
The affected areas include Longford Town, Newtown Forbes, Balinalee, part of Edgeworthstown, part of Drumlish, part of Killashee and surrounding areas.
All customers on this supply are advised to boil water before use until further notice.
Irish Water and Longford County Council are continuing with repairs onsite, conducting a flushing of the network and progressing with sampling of water supplies.
The Environmental Protection Agency will complete an audit of the water treatment plant this week.
Irish Water said drinking water experts are working with Longford County Council and the HSE to lift the notice "as quickly and as safely as possible".
In line with HSE Covid-19 advice and the requirement for frequent hand washing, Irish Water advises that the water remains suitable for this purpose and boiling the water is not required for handwashing.
John Gavin, Asset Operations Lead with Irish Water, said public health is the agency's "number one priority".
Vulnerable customers who have registered with Irish Water will receive direct communication on this Boil Water Notice and are reminded that the water is safe to consume once boiled.
Water in these areas must be boiled for drinking, drinks made with water, preparation of salads and similar foods, which are not cooked prior to eating, brushing of teeth, and making of ice.
Ice already in freezers should be discarded and ice should be made from cooled boiled water.
Water can be used for personal hygiene, bathing and flushing of toilets.
Water should be boiled by "bringing to a vigorous, rolling boil", for example with an automatic kettle, and allowed to cool.
It should be covered and stored in a refrigerator or cold place.
Irish Water said water from the hot tap is not safe to drink and domestic water filters will not render water safe to drink.
Caution should be taken when bathing children to ensure that they do not swallow the bathing water.
Prepare infant feeds with water that has been brought to the boil once and cooled.
Water that has been re-boiled several times should not be used.
If bottled water is used for the preparation of infant feeds it should be boiled once and cooled.