A massive plant with a capacity to launder 15 million litres of oil per year was uncovered in Co Monaghan today.
Gardaí and officers from the Revenue's Customs service discovered the plant in a commercial yard at Longfield in Castleblaney this morning.
It posed a potential loss of €7.5m to the Exchequer every year.
A man in his early 60s and two others in their mid-30s were arrested and are being detained at Carickmacross and Castleblayney Garda stations.