Garda Press say the seizure is part of Operation Tara which was set up to disrupt, dismantle and prosecute drug trafficking networks.
Gardaí from the Divisional Drugs Unit at Anglesea Street stopped and searched a man on Gratton Street in the city yesterday shortly after 6pm.
They found him in possession of suspected cannabis herb with a street value of around €6,000.
The man in his 30s was arrested and taken to the Bridewell Garda where he is currently being detained.
A follow up search of a house in Sunday's Well was carried out a short time later, where Gardaí found 15 kilos of suspected cannabis herb with an estimated street value of €300,000.
€15,500 of cash was also discovered and seized.
The drugs will be analysed by Forensic Science Ireland and Garda Press say investigations are ongoing.
