210 kilos of the drug were discovered in large steel containers within three pallets which came from Germany and were destined for an address in Dublin.
Revenue officers with the help of detector dog Waffle made the discovery today.
Meanwhile, 10 kilos of cannabis worth €200,000 were discovered in a separate search.
Revenue officers stopped and searched a vehicle which had arrived from the UK, and a man in his 30s has been arrested and is being questioned at a garda station in Dublin.
A statement from Revenue says: "Today, as a result of risk profiling and with the assistance of detector dog Waffle, Revenue officers seized 210 kgs of herbal cannabis with an estimated value of €4.2 million at Dublin Port.
"The illicit drugs were discovered concealed in large steel containers within three pallets originating from Germany and were destined for an address in Dublin.
