The European Court of Justice has found the Irish Government guilty of failing to introduce proper laws to protect shellfish areas around the coast, and failing to introduce proper pollution control programmes.
The action was taken by the European Commission following a complaint from the Irish Shellfish Association.
The case arises from a 1979 European Directive that required states to introduce national laws to protect areas where shellfish, like mussels and oysters, grow.
Ireland introduced regulations in 1994, but the Court of Justice found today that those regulations did not incorporate strict limits on the amount of pollutants like metals that were permitted in waters where shellfish are farmed.
The court also found that Ireland had not designated enough shellfish areas for protection, and had failed to introduce pollution control measures for those areas.
The court accepted all of the commission's arguments and awarded costs against Ireland.
This is the third environmental case Ireland has lost before the European court this year.
Last month the court found Ireland had failed to give the public adequate access to environmental information, as required under EU law.
In January, Ireland was found not to have complied with its obligations under a 1992 directive to protect and conserve natural habitats, flora and fauna.