Cocaine with an estimated street value of between €70m and €80m has been seized on board a yacht off the southwest coast.
Three men believed to be from the UK were detained on board the 18-metre yacht, the Makayabella.
The yacht tonight arrived on tow into Haulbowline in Cork.
A full garda technical examination of the vessel will get under way in the morning.
The seizure of around a tonne of cocaine is believed to be one of the biggest drugs seizures at sea in Europe this year.
The yacht was 200 miles off Mizen Head when the seizure was made.
It was boarded under cover of darkness in the early hours of Tuesday morning.
The crew are not believed to have put up resistance.
Lieutenant Commander Eric Timon of the Naval Service described the seizure as "extremely significant".
The operation was coordinated in Ireland by the Joint Task Force on Drug Interdiction, a government body set up to combat drug smuggling.
The operation involved the Naval Service, Revenue's Customs Service and gardaí. It has been ongoing for some time.
The Maritime Analysis and Operations Centre for Narcotics, based in Lisbon in Portugal, is also understood to have been involved in the operation, along with Britain's National Crime Agency, as well as security and customs agencies elsewhere.
The shipment is understood to have originated in South America, while the yacht set sail from the Caribbean.
It is believed to have been due to rendezvous or "cooper" with another vessel, although this has not been confirmed.
It is not known at this stage if the drugs were due to be landed in Ireland for movement to the UK or if they were due to be brought ashore in the UK directly.
Dozens of bales of cocaine are believed to have been concealed on board the yacht.
Details of the operation were kept secret to facilitate follow-up investigations, both in Ireland and abroad.
The vessel is believed to be in poor condition following its transatlantic crossing.