The Environmental Protection Agency (EPA) says nearly 300 septic tank systems that failed inspection pose a risk to human health and the environment.
A new report has found that over 50% of systems failed inspection in 2019.
While 27% of systems that failed inspections during 2013 to 2019 are still not fixed.
A lack of maintenance and desludging was identified as a key issue.
There are an estimated 500,000 septic tanks and other domestic waste water treatment systems in Ireland.
Noel Byrne, EPA senior inspector, told Newstalk Breakfast: "11,060 of the septic tanks that were inspected, half of those failed either because they were not built properly due to structural problems or not being maintained properly.
"More concerning really in there is that nearly 300 systems that failed posed a risk to human health and the environment, which is very concerning.
"So it's important that the public understand these findings today and the risks that go with them".
"There's 500,000 septic tanks in Ireland.... the EPA prepared the inspection plan for the country nationally - they are targeted to the areas of the greatest risk.
"So where you have problematic soil, so they are targeted in areas where you're more likely to find problems.
"And if they are problems then they could impact on their water quality or even impact on human health.
"It's important then, really, for the householder to be able to recognise if there is an issue how do I see it?
"And there is a few simple things for householders that they can do.
"I suppose the first key thing here... there's 165,000 households with a septic tank system and a drinking water well on the same site.
"Householders with private wells are vulnerable to pollution from a faulty septic tank, so they need to be aware of the risks."
"The simple thing really for a household that they need to do is if you have a well on your site with a septic tank, and if there's issues with your septic tank, get your well tested.
"It's a simple cost of around €65 to test for contamination."
"Similarly check around the septic tank area, particularly after the heavy rainfall, see is there any leaks or ponding in the area or if there's any discharges going to ditches.
"If you see anything like that, you know you have an issue with your septic tank".
Dr Tom Ryan, director of the EPA's Office of Environmental Enforcement, said: "If you do not maintain your septic tank, it can contaminate your own or your neighbour’s well, putting your health at risk and that of your family and neighbours.
"It may also pollute your local stream or river.
"You can take simple steps to maintain your septic tank by cleaning it out regularly and by making sure it is not leaking, ponding or discharging to ditches.
"The Government’s expanded septic tank grant scheme broadens the availability of grants and increases the maximum grant available which is welcomed", he added.
