Officers said a fuel laundering operation was "in progress" when they raided the premises at Knightstown near Kells in Co Meath at 8.30am.
Four men - two from Northern Ireland, one from Co Meath and one from Co Louth - were arrested at the scene.
Some 30,000 litres of laundered fuel and 12,000 litres of 'sludge', two tanker trailer units and four vehicles were seized at the plant along with ancilliary equipment.
Revenue officials estimate the plant had the capacity to launder 7.8 million litres of fuel per annum with a potential tax loss to the exchequer of about €4 million every year.