Just 4% of homeowners with a septic tank have registered it online since the Environment Minister introduced a scheme in June.
The government introduced new rules on septic tanks obliging homeowners to make sure they're working properly and not polluting water.
People who have a septic tank or waste-water treatment system are required to register them by February 1 next year.
Registration costs €5 until September 28, when the fee rises to €50.
According to the latest figures from the Department of the Environment, only 18,000 out of an estimated 497,000 properties have registered their domestic waste water treatment system.
The controversial plan to register septic tanks has been criticised by some rural dwellers who claim it discriminates against them and would force them to spend thousands of euro upgrading their tanks while city dwellers did not face such charges.