A major operation targeting illegal fuel operations and the activities of a Co Louth-based criminal gang has been carried out.
The Criminal Assets Bureau, along with gardaí and Revenue personnel, raided 19 premises in ten counties.
Houses, warehouses and business premises were searched this morning.
Computers, documents and bank statements were seized during the cross-border operation, which involved a number of Northern Ireland agencies.
The searches were carried out in Louth, Monaghan, Dublin, Kildare, Waterford, Offaly, Roscommon, Westmeath, Meath and Tipperary.
The operation was also supported by the Emergency Response Unit, Garda Air Support Unit and the Air Corps.
A garda spokesman said: "The objectives of the operation are to seize evidence of assets deriving from oil fraud and money laundering, seize and dismantle illegal oil operations, seize cash or other assets including vehicles used in the criminal activity and to freeze bank accounts".
He said investigations conducted by the CAB to date have identified significant funds being generated and laundered from the sale of laundered diesel - with the colour marker washed out - and stretched petrol, which has been diluted with methanol.
"The profits generated from this illegal activity are significant, as is the loss to the Irish Exchequer," gardaí added.
"This illegal activity has knock-on effects on legitimate businesses as well as on unsuspecting customers who have very often experienced damage to the fuel system of their vehicles from the laundered diesel or poor return per litre of petrol."
No arrests were made during the operation.
However, gardaí seized a large quantity of cash at two locations, along with business records and computers, which will now undergo forensic examination by CAB.
Gardaí also seized fuel-laundering equipment, which initial analysis indicates could have the capabality of processing hundreds of thousands of litres of diesel.
This could amount to 10 million litres a year, potentially costing the Exchequer €5.5m.
Gardaí say over 40,000 litres of what is believed to be laundered fuel was also discovered, along with 16,000 litres of toxic sludge; a waste product from the fuel-laundering process.
The CAB has frozen more than 25 bank accounts suspected to be linked to the operation.