Customs officers have uncovered two mobile oil laundering plants in Dundalk, Co Louth.
The plants at Ravensdale Park had the capacity to launder about 15 million litres of fuel per year with a potential loss to the Exchequer of €8m a year.
Three tonnes of toxic waste were found at one of the locations.
Removing the coloured dye from cheaper agricultural diesel and then selling it on as road fuel makes huge profits for those involved, but at a cost to the State and motorists.
This morning's two raids by Revenue and armed gardaí netted a total of 34,000 litres of fuel, as well as six vehicles and laundering equipment.
No arrests were made.
Another laundry was found in the garage of a house near Castleblayney less than a month ago as part of a nationwide search operation.
That could have washed enough diesel to cost the state €5.5m in lost revenue.
Revenue has said that mobile fuel laundering plants are becoming more common.
Speaking on RTÉ's News at One, Sean Kelleher, a manager with Revenue's Customs Service said that it has uncovered a number of mobile plants in the last few years.
Mr Kelleher said there is evidence to show that some criminals are moving their fuel laundering operations further south of the border.