Customs officers and armed gardaí have raided an illegal diesel laundering operation near Castleblayney, Co Monaghan.
A large scale operation capable of "washing" the dye from 15m litres of diesel every year was discovered during the dawn raid at a premises at Drumacon.
Revenue estimates it would have cost the State €8m a year in lost revenue.
One man was arrested and a number of vehicles were seized.
About seven tonnes of toxic waste was found at the scene.