That's according to a TU Dublin study.
One in five census areas detected infection cases caused by cryptosporidium over 10 years.
The Irish Times reports just over 3,400 out of 18,600 localities saw illnesses due to the common water-based parasite.
The data's included in a nationwide analysis into the bug, which causes gasto-intestinal issues.
Symptoms include weight loss, vomiting, diarrhoea, abdominal pain, nausea and fever.
Study reveals extent of challenges posed by common waterborne bug https://t.co/ijaIQOotNJ
— The Irish Times (@IrishTimes) September 3, 2021