The ESB could face criminal prosecution over the leaking of hazardous chemicals from underground power cables.
An Environmental Protection Agency (EPA) investigation found the company knew of 68 leaks of dangerous insulating oil over 25 years up to last summer but only told local authorities about 20 of them.
The ESB also failed to assess the impact of the leaks despite potential danger to the public and environment from contaminated water and soil.
"There have been clear failures in ESB Networks' reporting obligations to the relevant local authorities in respect of these incidents," the EPA said.
The investigation also found seven new leaks since last summer but under ESB's cable replacement plan, it will take until 2035 to remove all the defective cables.
An investigation was launched following an 'RTÉ Investigates' programme in which ESB employee turned whistleblower Seamus O'Loughlin revealed his repeated efforts to get the company to adequately address the leaks issue.
The ESB admitted at least 500,000 litres of the problematic mineral oil leaked from underground cables in five local authority areas between 1993 and June 2019.
The power company says it cannot find information on any leaks that may have occurred prior to 1993.
The affected areas are under Dublin and Cork City Councils and Wicklow, South Dublin and Dún Laoghaire-Rathdown County Councils.
But the particular fluid-filled cables at the centre of the affair are also in six other county council areas: Fingal, Cork, Limerick, Clare, Wexford and Kerry. Around 221km of these cables were installed between 1950 and 1989 and just 44km have so far been replaced. The EPA found that the ESB did provide figures for leaks in its annual environmental reports to the Commission for Regulation of Utilities in recent years but the commission's remit was to approve spending on replacement works rather than question the environmental implications.
According to the EPA's report, the ESB has committed to carrying out environmental impact assessments on all leak sites and has adopted new protocols for the alerting of relevant authorities.
However, the EPA says local authorities where leaks took place should now investigate with a view to initiating criminal prosecutions or other enforcement proceedings.
"It will be a matter for each local authority to decide the appropriate enforcement action to take.
"While EPA would not tell the local authorities what enforcement action to take, their performance in this regard will be a matter for review by EPA in future under our supervisory role," it said.
The ESB said: "We have co-operated fully with this investigation. We will carefully consider the report and don't intend to make any further comment at this time."