Customs officers and gardaí have raided a diesel-laundering plant in Co Monaghan.
The plant at Drumboat, Inniskeen had the capacity to launder 20 million litres of diesel a year.
That equates to a potential loss to the Exchequer of €10m in excise and duties.
Three oil tankers, a mobile oil laundry, two stationary tanks and 50,000 litres of laundered fuel were also uncovered.
A 42-year-old man was arrested in relation to breaches of environmental waste legislation and has since been released without charge.
The garda investigation is continuing and a file will be sent to the Director of Public Prosecutions.
The Revenue Commissioners suspect the plant was operating for more than a year.
It said organised criminal gangs are setting up front companies to source and launder fuel, which is then sold through petrol stations they control.
The commissioners estimate the gangs control 150 of these petrol stations all over the country.
The overnight operation was supported by armed gardaí.
Customs officials described the seizure as "significant".
