Two men have now been charged with manslaughter over the deaths of 39 people found in the back of a truck near London.
British police said on Friday, that Eamon Harrison, 23, from Northern Ireland, was accused of 39 counts of manslaughter as well as human trafficking and immigration offences.
The alleged truck driver, Maurice Robinson, 25, from Northern Ireland, has already been charged over the deaths.
Police have also appealed to two brothers suspected of being involved to hand themselves in to detectives.
Meanwhile, police in Vietnam, where many of the victims are believed to have come from, said they had detained two people.
The discovery of the bodies in a container on an industrial estate has shone a spotlight on the illicit trade that sees the poor of Asia, Africa and the Middle East embark on perilous journeys to the West.
Harrison appeared at Dublin's High Court at the start of proceedings to extradite him from Ireland to Britain. He was remanded in custody until November 11, a court spokesman said.
The bodies were found in the early hours of October 23 after the container arrived in Britain from Zeebrugge in Belgium. The container was picked up at Purfleet dock, east of London, by a truck allegedly driven by Robinson.
The victims were found not long afterwards. Police have not confirmed the exact cause of their deaths.
Police originally said the victims were believed to be Chinese, but Vietnam fears many were its citizens.
Vietnamese police said they had arrested two people and summoned others for questioning on Friday after opening a criminal investigation into suspected human trafficking.
British police also appealed on Friday to Ronan Hughes, 40, and his brother Christopher, 34, from Armagh in Northern Ireland, who they said were crucial to their inquiries. They are wanted on suspicion of manslaughter and human trafficking.
Daniel Stoten, the officer leading the investigation, said police had spoken by telephone to Ronan Hughes recently but needed to question the brothers in person.
"Today I want to make a direct appeal. Ronan and Christopher, hand yourselves in," Detective Chief Inspector Stoten said.
Lawyers for Global Trailer Rentals, the owner of the trailer, have said Ronan Hughes signed the papers to rent the container, giving an address matching the haulier, C Hughes Transport. Christopher Hughes is listed as a director of C Hughes Logistics Ltd, based in Armagh.