The Environmental Protection Agency has said that the quality of public drinking water in Ireland remains high, with 99.9% compliance with microbiological limits.
However, it said it was particularly concerned about poor operation practices at some water treatment plants and that the incidence of the cryptosporidium parasite in drinking water has increased in the past three years.
Ireland has 804 public water supplies serving 1.3 million households and this report said the quality of drinking water those households are receiving was very high.
It said 124,000 samples were tested for microbes, bacteria, chemical contamination, and for look, smell and taste.
The EPA said 99.9% passed the microbiological tests and 99.6% passed the chemical tests.
Yet, the EPA said it was particularly concerned that the incidence of cryptosporidium in Ireland's drinking water has doubled in two years.
Cryptosporidium is a parasite found in human and animal waste that can cause persistent diarrhoea.
It was found in 25 public water supplies last year and Irish Water has been instructed to take remedial action to improve treatment infrastructure and processes to stop it.
E. coli, another bacterium that causes illness, was detected in 12 supplies, while pesticide limits were exceeded in 34 supplies. Both of these are improvements from the previous year.
In all, there were 44 boil water notices and 15 water restriction notices affecting 112,000 people issued during 2018. That is five times more people affected than the previous year.
A failure of the disinfection system at the Vartry Reservoir caused 65,000 people in Dublin city to be put on a boil water notice early in the year.
Nitrate contamination led to water restrictions in Kiltegan in Co Wicklow and Ballyragget Co Kilkenny.
Chlorine dosing levels caused problems for households in Dunboyne, Co Meath and Shercock, Co Cavan.
Big problems were caused by a kerosene spillage into the water supply in Fethard in Co Tipperary and by the contamination of a spring source in Rathkeale in Co Limerick.
Extreme weather events also played havoc with water supplies last year
Large amounts of snow and the extremely cold conditions during Storm Emma left many water treatment plants inaccessible.
Precautionary boil water notices were issued. Many people had to be put on water restrictions, while others were left with no water at all.
That was followed by drought conditions in June and July and the first ever nationwide hosepipe ban, which lasted for three months.
The priorities for Irish Water include ensuring that all water treatment plants are effective in keeping the water free from bacteria and harmful chemicals; to eliminate harmful lead piping; prevent pesticides from entering Irish waters; and to manage the risks to public supplies.
Dr Tom Ryan, Director of the EPA's Office of Environmental Enforcement said: "We are seeing an upward trend in cryptosporidium contamination in drinking water supplies.
"We know that cryptosporidium can cause serious gastrointestinal illness, particularly in young children and the elderly, and the EPA has ensured that Irish Water has investigated each of these cryptosporidium detections.
"Irish Water must make certain that water treatment plants are properly and effectively operated to protect public health. Those plants without appropriate treatment for Cryptosporidium need to be prioritised for investment by Irish Water."
The EPA has added supplies to the EPA Remedial Action List, following its audits of drinking water plants.
Irish Water has to prioritise sites on the EPA Remedial Action List and develop action plans for improvements to be completed by set dates.
Andy Fanning, Programme Manager, EPA's Office of Environmental Enforcement, said "At the end of 2018, the number of supplies on the EPA's Remedial Action List had decreased. Unfortunately, that downward trend has been reversed in the first six months of 2019, when we added eight supplies to the Remedial Action List.
"These additions highlight that there are still significant problems at many of Ireland's water treatment plants, with the potential to harm people's health.
"The EPA is particularly concerned about supplies where we have seen poor operational practices at water treatment plants. Consumers must have confidence that their water supply is not just safe to drink today, but will also be safe in the long term."
In response, Irish Water has said a systematic monitoring programme where they look at where the risk is most likely to have cryptosporidium in the water supplies had been introduced.
Speaking on RTÉ's News at One radio programme, Mark Macaulay, Asset Planning Manager with Irish Water said "we've enhanced our monitoring programmes and off the back of that we're actually finding more instances of cryptosporidium. And the good news from that is that what its doing is its feeding back into our operational plans and our investment plans.
"So we're now able to, because we know about it, we're able to respond more quickly and make sure we put the correct measures in place. And that includes being able to engage quickly with the HSE, the EPA and our customers to provide advice if its required."