Gardaí have discovered four industrial-sized cannabis growhouses in counties Mayo, Kildare, Waterford and Louth this afternoon.
Over 6,000 cannabis plants, with an estimated street value of €5m, have been seized.
An industrial unit in the Blanchardstown area of Dublin has also been raided and equipment for setting up growhouses has been discovered.
Lighting and irrigation systems, along with compost, flower pots and other equipment necessary to set up a growhouse, were seized.
The operation began just after midday when officers from the Garda National Drugs Unit, along with local gardaí from the counties involved, began a series of raids on five industrial premises.
Four large scale cannabis growhouses, each with between 1,500 and 2,000 plants were found in Swinford, Newbridge, Tramore and Ardee.
Five men have been arrested and are now being detained under drug trafficking legislation.
Two men are being held in Ballina, two more in Tramore and the fifth suspect is being questioned in Drogheda.
Gardaí believe they are connected to two separate criminal gangs, one of which was not only operating its own growhouses but also setting them up for other drug trafficking.