Officials estimate the 30,000 litres of waste, found in 30 drums near Castleblaney, County Monaghan, on Tuesday night, would have produced 300,000 litres of ‘washed’ diesel.
When sold on as road diesel, the criminals would have earned about 40c a litre profit, giving them a profit of around e1.2m.
Kieran Duffy, executive engineer with Monaghan County Council, said: “This is a huge seizure, absolutely massive, by far the biggest seizure of waste diesel we have had here. We usually get seizures of 2,000 or 3,000 litres. The biggest ever before was 6,000-7,000 litres.”
He said oil laundering was a major problem in Monaghan and Louth.
He said that the council might have to pay €150,000 to dispose of the waste safely and transport it to a special facility in Germany.
The trailer carrying the 30 drums was originally stolen near Rostrevor in County Down last week, before being parked in a forest at a well-known beauty spot, Concra Woods, outside Castleblaney, a couple of miles from the border.
A garda spokeswoman said the owner of the trailer spotted it when he was in Castleblaney and alerted gardaí, who later contacted Monaghan Co Council.
Mr Duffy said that the drums were packed neatly on the trailer and luckily none of the toxic material leaked.
He explained that the waste was made up of sulphuric acid, dye from the original diesel and some of the diesel itself.
Mr Duffy also pointed out that washed diesel is very damaging to a car’s engine as it removes the lubricating fluid.