Detectives have smashed a massive multimillion-euro diesel laundering plant in one of the biggest cross-border operations of its kind in Britain or Ireland.
The Irish Republic's Criminal Assets Bureau (CAB), backed by 300 officers as well as military personnel on both sides of the Irish border, launched a wave of raids under the cover of darkness on almost 20 premises.
What is believed to be the country's largest illegal fuel business based at Hackballscross in Co Louth, and linked to a well-known ex-IRA chief, was at the centre of the investigation.
"This is the centre of all the activity - the command and control centre if you like - and all the tentacles of the supply chain spread out from there," a senior Garda source said.
"This is about following a money trail and we are talking about serious money here.
"There are tens of millions slushing around the place."
Twenty-five bank accounts have been frozen while computers, iPads, mobile phones, business records and 20,000 euro (£17,342) in cash have been seized as part of the operation.
Code-named Operation Loft, and several months in the planning, the mission in the early hours of this morning uncovered a plant at land which straddles the Irish border and which has the capacity to launder 10 million litres of fuel every year.
Crime gangs exploit the border by buying green or red diesel - cut price fuel for agricultural use - on one side before cleaning the dyes from it so they can sell it on the other side through a number of rented retail garage outlets.
The racket is estimated to cost the public purse hundreds of millions every year.
The operation at Hackballscross is believed to be capable of doing the Irish exchequer alone out of 5.5 million euro (£4.8 million) a year.
More than 40,000 litres of what is believed to be laundered fuel was found at the scene along with more than 150 bags of a cleaning agent branded as Bleaching Earth.
In addition, 16 tanks each containing 1,000 litres of sludge - which is the waste product resulting from the laundering process - and a 40ft container were also recovered.
A string of other premises including seven homes, 10 businesses - mostly filling station forecourts - and two warehouses were searched across the counties of Monaghan, Dublin, Kildare, Waterford, Offaly, Roscommon, Westmeath, Meath and Tipperary.
In Northern Ireland, three premises - both private and commercial - were searched by Customs officials backed up by PSNI technical support officers, while a helicopter was also tasked to the scene.
No arrests have been made but detectives said the operation was primarily about gathering evidence.
"This is about following the money and we will be doing that now," a senior source said.
While the mission was deemed a success, police believe they have not fully dismantled the country's largest illegal fuel trade network.
"We have disrupted it, we have hindered it but we don't believe we have fully closed it down," the source added.
"These are cunning, ruthless individuals who have been at this a long time.
"It is the biggest fuel laundering operation on these islands and the trade is endemic in that part of the country.
"But we are hitting them where it hurts - and that's the money."
Around 100 personnel from various agencies in Northern Ireland, including Her Majesty's Revenue and Customs (HMRC), along with more than 200 gardai and customs officers in the Irish Republic took part in the joint investigation.
Spearheaded by CAB - set up to target the assets of Ireland's organised crime gangs - it also involved Dublin's Special Detective Unit, the armed Emergency Response Unit, Garda Air Support Unit and the Air Corps.
A Garda spokesman said: "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."