The Commercial Court has ruled that the Electricity Supply Board was 60% liable for damage caused to University College Cork's campus from flooding in November 2009.
The court ruled that UCC was 40% liable due to contributory negligence.
UCC, on behalf of its insurer Aviva, had claimed ESB's actions and inactions in relation to releasing water from its two hydro-electric dams on the River Lee on 19 November, and in the early hours of the following day, were "highly dangerous" and led to significant unnecessary additional flooding causing substantial damage.
The court heard 30 acres of UCC's 80-acre campus were submerged under water and 29 campus buildings, several student accommodation blocks and a sports complex were damaged.
In a judgement of more than 500 pages handed down this afternoon, Mr Justice Max Barrett ruled that the ESB could and should have reacted to the weather forecasts it received from 16 November and spilled water in greater amounts than it did to create the space for more water at the reservoirs.
It should have maintained lower water levels in the reservoirs in the period leading up to 19 November, he found. He found the ESB's rules governing the discharge of water from the dams were not necessary or appropriate in the circumstances and this was recognisable at the time.
He also found the ESB failed to give anyone including UCC adequate or timely warning about the events it knew were happening.
Furthermore, he said ESB failed to undertake any adequate risk assessment exercise.
However, the judge also criticised UCC and said it failed to respond to at least 50 instances where it was put on notice of the flood risk to buildings it built or acquired on the Lee floodplain.
He said UCC's failure to act left its properties vulnerable and exposed its students and staff to significant hazard.
UCC claimed the ESB was liable for the damage caused to its campus. ESB claimed UCC was liable in contributory negligence. Judge Barrett said both were correct and he found both liable.
The case lasted 104 days and involved evidence from geologists and hydrologists.
A separate hearing is due to be held to determine damages, UCC is claiming around €19m.
ESB said its main priority was for the safety of people downstream of the dams. It said the inflow of water into the Lee catchment area during November 2009 was the worst since records began.
It also said the presence of the reservoirs and the manner in which the dam was operated by ESB meant that the flooding downstream of Inniscarra was less than it otherwise might have been.
A spokeswoman for the ESB said the board would be considering the judgment.
Cork Solicitor Joe Noonan, who represents some 40 householders and small businesses badly affected by the 2009 flood, has welcomed today's court decision.
Mr Noonan has said he hopes the ESB will confirm that they accept the ruling and do not seek to appeal it.
He said the decision was a long time coming and people are still suffering.