The Irish Wildlife Trust has expressed its "outrage" following confirmation of a fish kill in the River Glyde, Co. Louth.
More than 20,000 fish have died, including minnows, sticklebacks, salmon, eels, brown trout, roach, and pike.
Inland Fisheries Ireland (IFI) has confirmed that the source of the pollution was an agricultural discharge.
Commenting on the incident, Oisín Ó'Néill, Nature Advocacy Officer with the Irish Wildlife Trust, said: "What happened on the River Glyde is an environmental catastrophe and an absolute disgrace.
"More than 20,000 fish have been killed and a river ecosystem has been devastated.
"The source has been identified as an agricultural discharge. Those responsible must be held fully accountable and prosecution must follow.”
The Irish Wildlife Trust has said that Ireland's rivers "remain under intense pressure from agricultural pollution", particularly nutrient runoff associated with "intensive livestock farming".
The Trust has criticised the Irish Government for supporting a nitrates derogation despite "repeated warnings" from the Environmental Protection Agency (EPA) about declining water quality.
EPA assessments show that around half of Ireland's rivers and two-thirds of its estuaries are polluted.
Communities across Ireland have identified clean water as a top environmental priority, despite the Trust outlining that river quality continues to deteriorate.
An IFI report on fish kills between 1969 and 2022 found that agriculture accounted for 23% of all recorded fish kills nationwide.
“This [River Glyde incident is the predictable consequence of a system that continues to place the economic interests of intensive agricultural production ahead of the health of our rivers," Ó'Néill added.
"Ireland holds the shameful distinction of being the only country in the European Union still operating under a nitrates derogation, permitting higher levels of agricultural pollution than are allowed elsewhere in the EU.
"The Irish public overwhelmingly support clean water and healthy rivers.
"Yet they are forced to watch powerful agricultural lobby groups travel to Brussels to demand exemptions from regulations designed to protect our environment."
The Irish Wildlife Trust is calling on the government to take "decisive action" to tackle agricultural pollution and restore freshwater ecosystems.