A man in hi s 20s has been arrested after two oil laundering plants were uncovered.
The illegal operations were found on adjoining premises in a farmyard and a commercial unit in Killyneil, Silverstream, Co Monaghan.
Revenue said it was an intelligence-led operation.
The laundering plants had the capacity to process six million litres of fuel a year with a potential loss to the Exchequer of 3m euro.
One of the plants was a mobile operation and used bleaching earth while the static laundry, concealed in a container, used cat litter to launder the fuel.
Revenue said officers found 20,000 litres of oil, a lorry, a van, five bags of bleaching earth, a large quantity of cat litter and ancillary equipment were seized from the two sites.
Three tonnes of toxic waste were also uncovered.
The arrested man is from Northern Ireland.