Two women have been arrested as detectives seized cannabis plants with a street value of around half a million euro.
In a planned operation targeting the sale and supply of controlled drugs, gardai raided a business premises on the outskirts of Castleblayney, Co Monaghan, on Thursday evening.
Local officers were backed up by the Garda Regional Support Unit during the search.
Investigators said they uncovered "a grow house" or cannabis factory and seized plants valued at up to 500,000 euro.
A Garda spokesman said: "The location has been sealed off for a technical examination."
Two women, one in her 20s and one in her 30s, were arrested at the scene.
They are being held and questioned under the Criminal Justice Act at Carrickmacross Garda station.