At one point the boat rolled so badly that chains restraining 14 trucks with container loads snapped. As the larger vehicles shifted, most of the 33 cars on board were badly damaged. Passengers said crew members told them the vessel had rolled to within four degrees of sinking.
Scenes on passenger decks were described as chaos, with travellers thrown across the bar. The ferry righted itself and made its way to Holyhead unassisted, docking shortly after 4am.
The Stena-Sealink vessel Hibernia, carrying 278 passengers and 33 crew, was two-thirds of its way from Dun Laoghaire to Holyhead when it got into difficulties at 1.35am yesterday.
The company said the roll-on roll-off ferry had been hit by two freak waves. The first had turned the ship around and the second caused a sudden roll as the ship went into a deep trough.
One truck driver at Holyhead said crew had told him the ferry had listed 40 degrees, and said 'if it was 44 degrees, that was it - she was under'.
Fifteen passengers and five crew were taken to hospital in Bangor. Two were detained, one with a broken wrist and one with a suspected fractured rib.
A coastguard described sea conditions as 'horrendous'. But a Stena spokeswoman, Helen Galvin, said the force 8 to 9 gale expected at 11pm on Saturday was 'nothing exceptional' for the 8,000 ton ship. 'It was the freak waves that did the damage.'
Some meteorologists question the description 'freak waves', noting that they are usually associated with seas off south-east Africa between May and October. Attribution of past accidents to freak waves has led to controversy when ferry operators have cited Act of God exclusion clauses in resisting liability.
David Andrews, Irish Minister for Defence and the Marine, said his department would be in contact with British authorities over the circumstances. The ferry was expected to be in service today.
A woman died after the van in which she was a passenger struck a tree brought down by gales in Co Monaghan early yesterday.