Irish customs officials seized the 50 million euros (45 million pounds) worth of cigarettes from a cargo ship that had sailed from the Philippines to the port of Greenore, near the border with Northern Ireland.
Ten people were arrested in the joint operation of Irish and British authorities, the officials said.
"This smuggling attempt was organised crime on a global scale," said John Whiting, assistant director at Britain's Revenue and Customs service.
The vessel had been kept under surveillance as it was suspected that a large quantity of contraband cigarettes was concealed within the cargo disguised as 'animal feed'.
