In two operations on roads near the Border at the weekend more than 100 vehicles were "dipped" for the suspected use of doctored fuel.
The checkpoints were set up close to Carrickmacross and Clones in Co Monaghan. Several finds of illicit fuel were made with the use of a special mobile laboratory unit.
It was the first time gardaí and revenue authorities combined with social welfare officials in an anti-fraud operation.
Several passengers were asked by the officials for their identities and PRSI numbers by the roadside units.
According to senior gardaí in the Cavan/Monaghan Garda division, yesterday, the "fraud dragnet" was a combined operation by gardaí, customs authorities, The Health and Safety Authority, the Department of Transport, and the Department of the Environment.
A senior Garda officer said : "This type of crackdown was on the cards for a long time in view of the huge amount of doctored fuel being used in the Border area and further south.
"The number of detections made shows the volume of abuse and the extent of fraud involving people in the region. The simultaneous operation of these checkpoints was very effective and was introduced in the wake of recent swoops on illicit diesel laundering plants on both sides of the Border."
A regional revenue official said yesterday it is likely more similar operations will be organised to detect the use of illegal fuel.
"Perhaps this type of operation will act as a deterrent for those using the doctored fuel," he said.
"The whole operation is however aimed at targeting the use of illegal fuel along the Border and hitting a racket in which some ruthless racketeers are creaming off up to €50 million a year, if not a lot more".
He added that intelligence had also indicated a large number of small-time smugglers had recently moved into the fuel-doctoring scam in a bid to realise a fast profit from the sale of tax-rebated fuel at the higher price for which the non-coloured fuel is sold.
He added: "The revenue authorities are now able to determine, on the spot, thanks to a mobile laboratory unit which is used at checkpoints, if seemingly clear fuel has been doctored.
"We believe we are now close, after all these years, to blowing the whistle on the fuel racketeers."
A social welfare department spokesman confirmed yesterday that files were prepared in relation to a number of men who were stopped at some of the checkpoints on their way to work. "Prosecutions are likely if it is established these individuals were travelling out of their own area to work on building sites and other schemes while drawing unemployment benefit," he said.