A large fish kill in the Blackwater river in Cork cannot be explained, the final report on the incident has concluded.

Some potent substance killed 32,000-42,000 fish – mostly prized salmon and trout – last month but multiple agencies involved in the investigation cannot say what it was.

Seven days may have passed between the incident that led to the kill and the first tests of possible sources.

The report by the agencies says the substance likely entered the river as early as August 5th and dead fish were spotted on August 9th but the Environmental Protection Agency (EPA) only became aware of the situation on August 12th. That date was the first day the EPA took samples from discharge flows from industries near the river and its tributaries.

The subsequent investigation also involved Inland Fisheries Ireland, the National Parks and Wildlife Service, Cork County Council, Uisce Éireann, the Marine Institute, the Departments of Environment and Agriculture and the HSE.

Their joint report says dozens of industrial and commercial sites were inspected, multiple samples of water, fish and other river creatures were taken and 900 potentially damaging substances were tested for, but no conclusive results emerged.

“Despite the significant investigation by members of the inter-agency group, the pollutant or the source, that caused the fish mortalities has not been identified,” they said.

The most they could determine was an approximate time and location of the original incident.

“It may be concluded that a waterborne irritant likely entered the river Blackwater around 5/6 August, around 72 hours before the first mortalities were observed on 9 August 2025,” the report states.

[ ‘We took out 450 fish in an hour’: Blackwater pollution highlights decade of poor environmental complianceOpens in new window ]

This likely happened “at an unidentified point most likely upstream of the uppermost limit of Inland Fisheries Ireland observed mortalities (main channel between Gortmore and upstream of Roskeen Bridge 13 August)”.

However, it “dissipated quickly rendering it undetectable in water samples and fish tissue samples”.

Most of the dead fish were found around Mallow but dead and injured fish were found from Banteer, 22km upstream of Mallow, to Castletownroche, 17km downstream.

Anglers reported distressing scenes of dead and dying fish with multiple marks and lesions, swollen eyes and damage to their gills.

While the investigation identified no pollution source, the report says North Cork Creameries, the largest licensed facility near where the fish kill occurred, will continue to be closely monitored by the EPA after it found recent breaches of its licence.

[ Rules to protect Ireland’s fragile rivers are being repeatedly breachedOpens in new window ]

The report confirmed “non-compliances were detected in the wastewater treatment plant discharge from North Cork Creameries in the June to August period and were serious and entirely unacceptable”.

The licence breaches arose primarily due to a lack of organised management or control of wastewater treatment plant activities at the co-op, which discharges in the Allow, a tributary of the Blackwater.

The EPA also found “a lack of appropriate expertise to resolve significant operational issues, a failure to appropriately generate, manage, maintain and use critical data sets to inform corrective actions and a disregard for licence requirements and licence limits”.

“These compliance issues have not yet been fully resolved by the licensee, and the EPA is rigorously pursuing the enforcement of the licence breaches arising as a matter of priority and urgency, in line with its compliance and enforcement policy,” it added.

In April this year North Cork Creameries was convicted on eight counts for exceeding ammonia and nitrogen levels and fined €11,000 in a case brought by the EPA.

Minister of State with responsibility for fisheries Timmy Dooley said the investigation had been “exhaustive” and the findings provided reassurance that the incident was “a short-lived event, with no evidence of ongoing pollution risks”.

Former lord mayor of Dublin Nial Ring was told he could no longer let a rental property he owns because of a failure to maintain the house in a proper state of structural repair.

Dublin City Council, in deciding to issue the notice, listed 10 contraventions of the regulations set out in the Housing (Standards for Rented Houses) Regulations 2019, according to a statement.

The house, 7 Caledon Court, East Wall, appears on the current list of 156 properties against which the council has issued prohibition notices as part of its role in monitoring standards in the privately rented residential property sector.

Prohibition notices are only issued after a landlord has been issued with an improvement notice but has failed to implement the improvements sought.

Improvement notices are issued after the council has inspected a property, having received a complaint or having inspected the property for some other reason.

In his declaration of interests last year, Mr Ring, an Independent north inner city councillor, said the East Wall house was being rented to a tenant availing of the Housing Assistance Payment (HAP).

The contraventions of the regulations noted by the council involved failing to keep the house in a proper state of structural repair, the failure to put safety restrictors on windows through which a person could fall to the ground, the condition of the sanitary facilities, the installation and maintenance of the heating system, the absence of a carbon monoxide alarm, the condition of the kitchen facilities, the absence of a fire alarm and fire blanket and the condition of energy supply installations.

[ Former lord mayor Nial Ring told by council he cannot let Dublin rental propertyOpens in new window ]

The Caledon Court house is one of several modest homes in a small, gated development on the East Road in Dublin 3, most of which appear on the register of rental properties maintained by the Residential Tenancies Board (RTB). Number 7 is not on the register.

In a text message, Mr Ring said the property was no longer being rented through the HAP scheme so “has nothing to do with DCC or RTB since last March”.

In his declaration of interests, Mr Ring said he owned the Dublin 3 property and a 50 per cent share in 70 Ballybough Road, Dublin 3, a commercial building with a pub and overhead offices.

He also declared shares in IMC Exploration group Plc (IMC), a mining and exploration company with its registered address at 70 Ballybough Road.

IMC’s shares are quoted on the London Stock Exchange and it is exploring becoming dual listed on an exchange in Armenia, according to a recent statement.

Earlier this year, Mr Ring and his long-time business partner Liam McGrattan had charges against them struck out when they appeared in the Dublin District Court after being found intoxicated above the Ref Pub at 70 Ballybough Road during the early days of Covid-19 restrictions in 2020.

Both men were directors of IMC from 2011 to 2018, company records show. A report filed earlier this year shows Mr Ring then owned slightly more than 1 per cent of the company’s shares while a company owned by Mr McGrattan, Wilhan Ltd, owned another 2 per cent.

The largest shareholder, Prague-based Mineral Ventures Invest spol. s.r.o, owned 51 per cent. IMC has a market capital value of approximately €5 million.

Mr Ring is a former member of Fianna Fáil and a former political ally of Bertie Ahern. He was a government appointee to the board of the Industrial Development Authority for 10 years, standing down in 2008.