Around 88kg of heroin, worth more than €12 million, has been seized at Rosslare Port in Co Wexford.
Revenue officers, with the help of two sniffer dogs, found the drugs as part of an intelligence-led operation.
A truck and low-loader trailer, arriving into the Republic from mainland Europe, was stopped and searched on Thursday afternoon.
The drugs were found hidden in machinery being carried on the low-loader.
Revenue investigator Nick Gilligan said the heroin seizure was part of an international investigation.
He told RTÉ that Irish customs and government bodies in other EU states had been looking at the movement of illicit drugs in plants and machinery.
"We profiled such movements coming into Ireland and moving out and in conjunction with our international colleagues we had highlighted and profiled this particular consignment coming in," he said.
Officers used detection dogs Sam and Flynn and X-ray scanners to examine the equipment.
The drugs were found deep within the machinery.
Mr Gilligan said the drug haul was too large to be consumed in Ireland.
"At least a significant amount of this load would have been moved on somewhere else," he said.
Investigations are continuing into where some of the drugs were headed.
Mr Gilligan said he is hopeful of further drug seizures.
