Customs officers say the latest diesel laundering operation discovered last night could have cost the State up to €5m a year in lost revenue.
The find was made at a premises close to the N3 near Virginia in Cavan.
A lot of effort had gone in to concealing this diesel laundering operation. A yard behind a house had been lowered by over a metre to hide the illegal activity.
The premises at Whitegate had been under observation for sometime.
When customs officers raided it last night, they found two vans as well as equipment and bleaching earth used to remove the colour from cheaper agricultural diesel which is then sold on as road fuel.
This fuel is often sold cheaper than legitimate fuel, but it does cause major damage to engines.
One man found at the scene was questioned and subsequently released.
Customs officers estimate that the laundry could have washed around 10 million litres of diesel a year.