An organised crime gang operating in the North was behind the importation of herbal cannabis worth £1.3 million, police have said.
Three pensioners were among six people arrested after the haul was found on a shipping pallet inside an industrial sized fuel tank at a property in Dromore, Co Down on Friday.
The seizure is one of the biggest in the region in recent years.
Detective Superintendent Bobby Singleton said: "Cannabis is the backbone of the drugs economy and it is a significant profit maker for organised crime groups.
"Along with the potential harmful effects that it can do to individuals the violence, intimidation and drug death that accompanies the drug trade in Northern Ireland means that taking this load off the street has made a significant contribution to keeping people safe."
Two 68-year-old women and a 72-year-old man have been released on bail pending further inquiries, while two men, aged 32 and 37, and a 42-year-old woman remain in custody.
The discovery was made during a joint operation by the Police Service of Northern Ireland, An Garda Siochana and Irish customs officials as part of the Cross Border Joint Agency Task Force, established under the Fresh Start Agreement.
It followed intelligence sharing by law enforcement agencies on both sides of the border.
The PSNI has previously said there are between 60 and 70 organised crime gangs in the drugs trade in Northern Ireland.
Investigations are continuing but police have ruled out paramilitary involvement at this stage.
An Garda Siochana Assistant Commissioner Barry O'Brien said: "I wish to commend all agencies involved in both jurisdictions which resulted in the seizure of a substantial quantity of controlled drugs.
"This demonstrates the practical application of the Cross Border Joint Agency Task Force."
PSNI Assistant Chief Constable Stephen Martin added: "Our thanks go to all those involved in this operation for their diligence and professionalism.
"Their combined hard work and dedication has gone a long way in helping us achieve our shared goal of keeping people safe."
Meanwhile, suspected cocaine, heroin and ketamine estimated at over €2.5m was seized during a joint operation involving Gardai and customs officials at Dublin Port.