Gardaí believe that a gang with strong links to dissident republicans is behind a sophisticated cannabis-cultivation operation discovered near Grangebellew, Drogheda, on Tuesday night.
Four men were arrested when plain-clothes gardaí from Drogheda, Co Louth, raided the premises.
The men, all in their late 30s and known to gardaí, were arrested and detained under the Drug Trafficking Act.
Around 450 plants were found inside a large polytunnel similar to the type used to grow mushrooms.
Forensic scientists took samples of the plants yesterday, and informed sources said they were fully grown and ready for harvesting.
Some of the plants had been loaded on a lorry to be taken to another location where they would have been processed and prepared for sale as "grass". The drug would then have been sold by weight.
Gardaí have been targeting this gang for some time, and confirmed that the operation was intelligence-led.
Elaborate heating and irrigation systems had been installed inside the tunnel to create the ideal growing environment. Bamboo canes were used to support the plants.
This is the first such cultivation operation to be discovered on this part of the Border, and gardaí suspect that the "grass" would have been sold to drug dealers in the area.
Garda sources confirmed that the gang has strong links with dissident republicans, and has been involved in a number of high-profile robberies and hold-ups in the Border area in recent years.