Customs officers backed up by armed gardaí have seized a large diesel-laundering plant that was being used to clean marked diesel which was then smuggled across the Border.
The raid followed a surveillance operation by customs officials based in Dundalk. As well as 80,000 litres of diesel at various stages of being "washed", they also found 12,000 litres of waste residue.
They also seized a tanker, six large tanks and a curtain-sided truck at the plant at Drumkeith, Iniskeen, between Dundalk and Carrickmacross.
The plant, which had been operating since the start of the year, was laundering green diesel, which is legally sold in the Republic for agricultural use.
Louth County Council is investigating whether the waste residue recovered has caused any environmental pollution.
The plant was about five miles from the border with south Armagh. It is similar to the discovery last year of the largest such plant found in the Republic, in that both operations had automated systems installed so they could be operated effectively by remote control and therefore make the job of catching those behind it more difficult.
Garda sources say they believe subversives were behind the operation, but it is not yet clear if they are Provisional or dissident republicans. No arrests have yet been made.
The Revenue Commissioners say the plant had the potential to launder 50,000 litres of oil a day and the loss of the revenue in duties due on the fuel had the potential to run into millions of euro per year.
It is believed the plant had been operating since the start of the year.
A customs source said they were following a "definite line of enquiry" in relation to who was behind the operation and it is anticipated that a file will be sent to the Director of Public Prosecutions.