Sail training ship Asgard II is believed to have sunk after hitting an underwater object, a marine accident report has revealed.
Investigators found debris was common in the area off France where it suddenly went down on September 11, 2008.
It said there was slight decay in planks on the hull but no rot and no structural problems.
The Marine Casualty Investigation Board (MCIB) said there was anecdotal evidence suggesting that flotsam from ships was found in that region of the Bay of Biscay.
It had been speculated that the ship went down after a collision with a semi-submerged ship's container or a whale but the report found no specific evidence.
Tony Killeen, Defence Minister, said while the findings were inconclusive investigators said Asgard II was well-equipped and maintained with higher safety standards than required. However, one life raft had to be abandoned after the floor gave way and trainees on board had to transfer to another vessel.
Crew, including two trainees, reported strange noises below deck before the Asgard began rapidly taking on water and the captain ordered abandon ship.
The Boatswain, who had sailed on the boat for nine years, said she woke at about midnight hearing a knocking sound. She said it was an unusual noise to hear on a wooden ship at sea but she was not exactly sure what she heard. Her cabin was near where the major flooding took place.
An Asgard trainee reported a sudden noise around that time while another reported a series of loud bangs but not immediately after one another.
There were 20 trainees and five crew on board the Asgard when the alarm was raised. Many praised the actions of the ship's Master, Colm Newport, and the handling of the emergency. No-one was injured in the incident.