Drinking water supplies in 13 counties was found to have a high level of an environmental pollutant, which is linked to cancer.
A total of 59 samples failed Irish Water's inspections this year.
Trihalomethanes are usually formed as a result of adding chlorine to water.
Some studies suggest long-term exposure to high levels of THMs may be linked to cancer.
Irish Water tested 791 samples of drinking water this year to see if they had under the allowed limit of the environmental pollutants.
There were 59 failures across 13 different counties, with several in counties like Donegal, Clare and Louth.
However, many supplies had multiple failures.