An oil laundering plant that could have cost the state around nine million euro a year has been shut down.
Some 30,000 litres of illegal fuel were seized at the site in Tatebane, Hackballscross, Dundalk, Co Louth.
Three men were arrested in the operation by officers from Revenue's customs service and gardai.
The men, two from Northern Ireland and one from Co Louth, were taken to Dundalk Garda Station.
Three lorries, a tanker and other oil laundering equipment were seized at the plant, which had the capacity to launder around 18 million litres of fuel a year.
Officers also recovered 36 tonnes of toxic waste and spirits on which Irish duty had not been paid.
A Revenue spokesman said the operation was part of an ongoing crackdown on fuel laundering.
He also warned motorists against buying illegal fuel.
"Revenue would like to remind motorists and the public that they should be aware of the risks posed to their vehicles by using laundered fuel," said the spokesman.
"It also takes much-needed funds from the Exchequer, hurts legitimate trade and of course it is funding criminal activity."