Customs officers and gardai uncovered the plant in a commercial premises in Portlaw, Co Waterford.
A Customs spokeswoman said the plant has the capacity to launder a total of 10 million litres per annum, with a potential loss to the Exchequer of approximately five million euro per annum.
A number of large tanks and ancillary equipment have been detained.
"Investigations are on-going," she added.
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