More than 10 million cigarettes have been seized at Dublin Port.
The Gold Classic brand cigarettes were discovered in an unaccompanied 40ft container which arrived on a ferry from Rotterdam.
Officers from Revenue's Customs Service believe the haul has a street value of 4.6 million euro and a potential loss to the exchequer of 3.7 million euro.
A spokeswoman said investigations were ongoing.
"This seizure is part of Revenue's ongoing operations targeting the supply and sale of illegal cigarettes in the shadow economy," she said.
"If businesses or members of the public have any information regarding the smuggling or sale of illegal cigarettes or tobacco, they can contact Revenue in confidence."