Cryptosporidium was detected in 25 public water supplies last year - up from 17 the previous year.
However, the quality of drinking water in public supplies remains high - that's according to the EPA Drinking Water Quality in Public Supplies Report for 2018.
The report says that, while the overall quality of water supplies remains high, it's important Irish Water ensures that water treatment plants are properly and effectively operated to protect public health.
The Environmental Protection Agency says plants without appropriate treatment for cryptosporidium need to be prioritised for investment by Irish Water.
During 2018, 44 boil water notices were in place - this is now down to 19 boil water notices affecting over 15,000 customers.
The largest is the Lough Talt supply in Sligo, where over 12 and a half thousand customers are still boiling water before use.
However, work is due to start shortly on the long-awaited upgrade works at the Lough Talt water treatment plant, and the €10 million project should take between 12 and 14 months to complete.