A massive oil laundering plant with the capacity for an estimated 10 million litres of fuel a year has been uncovered by Customs officers.
Some 40,000 litres of laundered diesel, a lorry, four cars and a van were seized during the planned dawn raid near Monaghan town at 5am on Friday.
Ancillary equipment was also dismantled on the farmland in Silverstream, about four miles outside the town, and taken away for examination.
Four men, two in their 50s and two in their 20s, have been questioned in relation to the find but no arrests have been made.
Environmental officers are currently searching the site and surrounding areas for toxic sludge, a hazardous chemical by-product of the laundering process.
A Revenue spokeswoman said the fuel had the potential loss to the Exchequer of approximately six million euro a year.
Gardai from Monaghan and the Regional Response Unit supported officers from Revenue's Customs Service during the operation and investigations are ongoing.
This is the third large oil laundering plant uncovered by Customs so far this year. Last month an underground plant with the capacity to launder an estimated 7.5 million litres of fuel a year was discovered in a yard near Hackballscross, Dundalk, Co Louth.