GARDAI have uncovered the biggest cannabis growing operation in the State and seized more than 3,000 plants.
Officers arrested three Chinese nationals, but believe an Irish organised crime gang is behind the multi-million euro operation in Co Offaly.
The growhouse was located in a converted warehouse at Aughnacarna, Durrow, Tullamore, and plants in varying stages of growth were found there.
The 15,000 sq ft warehouse was built originally to hold spare parts for washing machines but became vacant last year. It was rented out on behalf of the cannabis gang shortly before Christmas.
Last night detectives from the garda national drugs unit and local gardai were trying to establish the identity of the major players.
Gardai said the growhouse had not yet yielded its first crop.
The growhouse was uncovered as a result of an intelligence-led operation involving members of the Tullamore drug unit and other local personnel.
During the searches on Tuesday night and yesterday morning, gardai seized a total of 3,001 plants, with an estimated street value of €1.2m.
Officers said it was the most sophisticated growhouse on the island and contained 10 separate chambers, with wooden panelling with a further double Perspex chamber at the top to allow the cannabis to be removed and the fumes extracted without attracting attention.
They said the "gardeners" had also been using very high quality nutrients and compost.
The three suspects, two men in their 20s and one aged 30, were being held last night at Tullamore garda station. They can be held for seven days.
The three were found in specially built living accommodation within the warehouse.
Over the past year, Operation Nitrogen has resulted in the seizure of 25,000 cannabis plants, which have a street value of €10m.
One officer pointed out last night: "This is only the amount we have seized. So if you look at the overall production line, you are talking about a multi-million euro enterprise with a lot of profit."
Gardai said the number of plants also indicated the level of demand here for the drug. It's estimated that the majority of the herbal cannabis being produced is destined for the street market in the Republic with the rest going across the Border.
After initial start up costs of between €5,000 and €9,000, the growers can show a profit of around €170,000 from an eight-week crop and many are developing a multi-crop cultivation programme.
This guarantees a fresh crop every four weeks, maximising the profit.
Gardai said cracking down on cannabis cultivation would remain a priority this year.
Late last year detectives were about to raid the biggest growhouse in the State near Rathangan, Co Kildare, when it was accidentally set on fire and destroyed.
Meanwhile, gardai seized herbal cannabis with a street value of over €350,000 following two related searches in Dublin.
The initial find was made by officers from the Dublin regional traffic department when they stopped a van on the M1 at Lissenhall, Swords. they uncovered 29 kilos of cannabis, worth €347,000.
In a follow up operation gardai found 40 cannabis plants worth €16,000 growing at a house in Tallaght.