A man is being questioned after the discovery of a large fuel laundering plant in Co Monaghan.
Customs raided the operation near Carrickmacross as hundreds of litres of oil were being illegally prepared.
Officers say the plant had the capacity to launder an estimated three million litres of fuel a year, costing the Exchequer about €1.8m.
Customs impounded 12,000 litres of laundered fuel and a truck.