The seizure was made over the weekend.
Gardaí in Carrickmacross, Co Monaghan discovered a grow house at a residential premises on Coolshannagh Road, Monaghan on Saturday, December 1st 2018.
During the course of the search a grow house was discovered which had over 320 cannabis plants growing at a mature stage of growth.
The plants have an estimated street value of €256,000
The premises was sealed off for a forensic technical examination, no arrests have been made and investigations are continuing.