A crackdown on fuel laundering and smuggling saved the state up to €200m in 2014, Revenue has said.
The Revenue Commissioners estimated the value of the blitz on ex-IRA gangs and border criminals following a report from the Comptroller & Auditor General that the illegal trade had been virtually wiped out.
Chairman Niall Cody said customs officials have been running comprehensive and successful strategies to shut down the underground operations and stop washed diesel getting on to forecourts.
"The tax loss associated with fuel laundering is not quantifiable but there's no doubt that it was very significant," he said.
Mr Cody said analysis of the oil market in Ireland in 2014 compared with 2013 pointed to the successes of Garda, customs and crime agencies in the Cross Border Fuel Group, tougher licensing regimes and the introduction of a new diesel marker agreed with the UK.
He said the review estimated up to €200m had been saved.
Mr Cody said a series of initiatives taken since 2011 "resulted in major strides (being) made to reduce and eliminate fuel laundering".
A report by the Comptroller & Auditor General at the end of last month noted a survey of filling stations which Revenue said was "authoritative evidence" that the multimillion-euro trade is close to being put out of business.
A Revenue official described the criminality as being "effectively eliminated".
An independent report estimated that the exchequer lost €239m through illegal fuel laundering last year.
Since 2008, it also cost €6.7m to clean up more than 1,000 dumps of toxic sludge from illegal fuel laundering plants, nearly all of which were in Louth and Monaghan.
Mr Cody warned that the gangs involved in fuel laundering in years gone by were moving into other black market operations including tobacco, drink and designer fuel which includes additives.
A fuel laundering plant was discovered in sight of the border last week, the Revenue chairman said.
"It is not eliminated on the island but we are very satisfied that we have practically eliminated it on this side of the border," Mr Cody said.
"There's no doubt that the people who were involved in organised excise fraud, they have not stopped doing something. We are very interested in looking at drink."