Half of all septic tanks inspected by local authorities last year failed inspection, according to the Environmental Protection Agency.
More than 1,100 inspections were completed last year.
The EPA reported that half of the septic tanks (560) failed inspection. One-in-five, or 230, were deemed to pose a risk to human health and the environment.
Structural defects and lack of maintenance were the most common reasons for failure.
Households are notified when they fail the inspection, advised that they must be fixed, and told of repair grants of up to €5,000 available in certain circumstances.
The EPA said local authorities need to increase enforcement where it is warranted so that environmental and public health are protected.
It is particularly concerned that the number of notified septic tank failures left unfixed by householders for more than two years has increased five-fold since 2016.
By the end of last year, 550 notified septic tank failures had been open for over two years.
This is five times more than in 2016, with half of these involving sewage surfacing in gardens and in many cases discharging illegally to ditches and streams.
The EPA said that enforcement of advisory notices by local authorities is inconsistent around the country.
The lowest rates of "failures fixed" are in counties Roscommon, Waterford, Leitrim, and Tipperary.
Local authorities have taken 47 legal actions for failure to fix septic tanks in 2013, with 11 in 2022.
The vast majority of these cases (90%) were taken by Wexford, Kerry, and Mayo county councils.
Nearly half a million households in Ireland have septic tanks to treat their sewage.
When they are faulty they can leak, potentially contaminating household drinking water wells with harmful bacteria and viruses.
They can also pool on garden surfaces, running off into ditches and streams, damaging the environment and putting people's health at risk.
This is why they are subject to inspections by local authorities, overseen by the EPA.
EPA Programme Manager Noel Byrne said the agency needs to see that the inspections are followed through and brought to a conclusion.
Speaking on RTÉ's Morning Ireland, he said: "There were 1,100 inspections completed last year by local authorities, but ... half of these failed inspection and fixing the faulty septic tanks is taking too long in many circumstances and that's continuing to risk people's health and the environment."
He said there is a grant of up to €5,000 available for people - administered through their local authority - if they have an inspection and a problem is detected with their system.
Natural Environment Officer at An Taisce Dr Elaine McGoff said excess nutrients like nitrogen and phosphorous from septic tanks can cause an overgrowth of plants and algae which can in turn clog up water courses and use up oxygen harming aquatic life such as insects and fish.
Speaking to RTÉ's Today with Claire Byrne, she said it can also lead to toxic algae blooms which can sometimes kill animals.
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Dr McGoff said it can also have a really significant health impact.
She said: "This is particularly worrying in an Irish context when we have 120,000 private wells in Ireland and research has shown that 70% of those are contaminated with E.coli."
Dr McGoff said Ireland has one of the highest levels in the EU of a really toxic strain of E.coli called VTEC which comes largely from septic tanks and cattle manure.
She said: "In the most severe cases, this can be potentially deadly. It can lead to kidney failure, brain damage and death and it's a particular risk to children and the elderly.
"Over 40% of our wells are potentially contaminated with VTEC so this has really far-reaching consequences.
"Having potentially a quarter of a million failing septic tanks should be setting off all the alarm bells."
Some people do not even know where their septic tank is, let alone what maintenance is required, Dr McGoff said.
She added a public awareness campaign is needed and consideration should be given to mandatory servicing contracts for septic tanks built into planning permissions.
The National Water Forum is also calling for an awareness campaign.
Speaking later to RTÉ's News at One, its chairperson Dr Matt Crowe said one of the most common problems with septic tanks is that they are not being "desludged".
He said that because septic tanks are under the ground, "people probably just forget about them when they're there until somebody comes knocking on the door doing an inspection".
Dr Crowe said if a secptic tank is working fine, there should not be any particular problem with it.
But if the tank is not working fine, you could have water pollution, with ground water or surface water being polluted, including your own water if you happen to have your own private well.
That is the public health risk, he said, and the environmental risk is the water quality, but that the two "tend to be connected".
He said a grant scheme is available for people to clean, upgrade or replace their septic tank and the NWF would like more promotion of the scheme.
Dr Crowe said the scheme is specifically linked to households which have already had an inspection or households which are in what are called "priority areas for action for water quality improvement".
He said grants were also available for upgrading and improving private wells and "it would be good to see a lot more promotion going on about those too".
