DAWN raids were carried out on almost 20 premises as 300 officers from north and south of the border targeted a criminal gang.
Computers, documents and bank statements were seized during the cross-border operation led by the Criminal Assets Bureau.
Operation Loft centred on the activities of a prominent Co Louth republican and an organised crime gang, who are suspected laundering and trading illegal fuel.
"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," said a garda spokesman.
Seven homes, 10 businesses and two warehouses were searched across the counties of Monaghan, Dublin, Kildare, Waterford, Offaly, Roscommon, Westmeath, Meath, Tipperary and Louth, where one property spanned across the border.
Gardai said 100 personnel from various agencies in Northern Ireland, including Her Majesty's Revenue and Customs (HMRC), and more than 200 gardai and customs officers in the Republic of Ireland took part in the joint investigation.
A senior garda said a prominent republican is at the centre of the multi-agency probe into the profits being generated from the illegal fuel trade.
"He is the centre of the all the activity and it all spans out from that like a spider's web," he added.
"We are collecting evidence to build up a case and are following the money."
He said the case is not linked to the murder of Detective Garda Adrian Donohoe, who was shot dead near Dundalk in January.
The raids, which began at 5am, were carried out at addresses and business premises of key members of the organised criminal group.
It involved members from CAB, the Special Detective Unit and various regional uniform Garda units, who were supported by the armed Emergency Response Unit (ERU), Garda Air Support Unit and the Air Corps.
HMRC and PSNI specialist teams searched locations in Northern Ireland, close to the border.
A Garda spokesman said investigations conducted by 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," gardai 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."
Garda later confirmed that no arrests were made during the operation.