The haul is expected to at least match last year's record €440m seizure at Dunlough Bay in west Cork. The drugs were on a vessel that was taken into custody around 200 miles off the southwest coast last night.
The boat was being brought to shore in west Cork under armed guard.
The Minister for Defence, Willie O'Dea congratulated the Naval Service on the success of this operation.
He said: 'It is a timely reminder to us all of the dangers the Naval Service confronts in the assistance it gives to the gardaí and Customs Assets in respect of drug interdiction.
'It is also clear evidence that Ireland has a Naval Service that is a multi-skilled, highly-trained force.