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 Criminal Assets Bureau (CAB), backed by 300 officers as well as military personnel on both sides of the 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.
A senior Garda source said: "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. 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 in cash were seized as part of the operation.
Code-named Operation Loft, and several months in the planning, the mission in the early hours of Wednesday 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 to sell on the other side through rented retail garage outlets. The operation at Hackballscross could the exchequer an estimated 5.5 million euro a year.
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 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."