Irish Water customers in Sligo have been advised that a boil water notice which was issued on 11 January is to remain in place.
It follows the detection of cryptosporidium during routine sampling at a water treatment plant. The decision was taken following consultation with the Health Service Executive and Irish Water as well as Sligo County Council.
Around 13,000 people are affected.
The towns where the notice applies are Tubbercurry and Ballymote, a large rural hinterland including the villages of Annagh, Aclare, Bellaghy, Curry, Lavagh, Ballanacarrow, Carrowneden, Kilmacteige and Coolaney. The areas are served by the Lough Talt Public Water Supply.
The Boil Water Notice also includes consumers supplied by the Ogham Group Water Scheme and the following areas in Co, Mayo: Cloontia, Doocastle and Quarryfield.
All persons served by these schemes are advised to boil their water before drinking, preparing food, brushing teeth and making ice.
Irish Water has said the water can be used for personal hygiene, bathing and flushing of toilets but caution should be taken when bathing children.
Cryptosporidium is a microscopic parasite that is found mainly in the faeces of infected humans or animals.