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.