Two mobile fuel laundering plants have been discovered close to the border.
One of the illegal operations was hidden in a curtain-sided lorry on commercial premises in Ravensdale Park, Dundalk, Co Louth, while the second was uncovered nearby in a slurry tank.
The plants were shut down during early morning raids carried out by officers from Revenue's Customs Service and armed gardai.
Revenue said that together, the plants had the capacity to launder about 15 million litres of fuel a year and cause a potential loss to the Exchequer of 8 million euro a year.
The first, a mobile laundry, which was processing fuel when uncovered, was parked at a business premises.
Revenue impounded four lorries, one of which had a concealed tank for transporting fuel, three oil tankers, 28,000 litres of fuel and ancillary equipment.
Three tonnes of toxic waste were also uncovered by officials at the scene.
Revenue said that in a connected search of a private residence and adjoining yard, a slurry tank had been converted into a mobile laundry.
One hundred bags of bleaching earth used to take dye out of fuel and 6,000 litres of fuel were seized along with three cars, two jeeps and one van on the site.
Revenue said investigations were ongoing.