A laundering plant and 25,000 litres of black market fuel have been seized by customs officers near the border in the North.
Tanks, three vehicles and filtration and storage equipment were also confiscated by Revenue and Customs.
The agency said the sophisticated illegal operation near Keady, Co Armagh, could cost the taxpayer £3.3m (€5m) a year. A large quantity of toxic chemical byproduct was also discovered.
Head of detection in the North Maggie Eyden said: “This waste is a danger for the local community and the environment, and is difficult and expensive to dispose of safely.”
A total of 10 tonnes of contaminated sludge, the hazardous chemical residue of the laundering process, were cleared from the site. The facility could produce 105,000 litres of laundered fuel a week.
Ms Eyden added: “People need to be aware of the potential environmental damage that can be caused by the indiscriminate dumping in our countryside of the waste products from the laundering process.
“They need to consider what happens to the waste byproduct and the damage caused by contamination to arable land, our water and rivers.”
Her officials were supported by the PSNI.