Co Armagh man Maurice Robinson, 25, has been charged with 39 counts of manslaughter over the deaths of 39 people found in a trailer in Grays, Essex, on Wednesday.
He is charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration and money laundering.
Robinson, who is known as Mo and is from Northern Ireland, was arrested shortly after the bodies of eight women and 31 men were found in the refrigerated trailer in an industrial park in Grays in the early hours of Wednesday.
The charges came after a man wanted by Essex Police as part of the investigation was arrested at Dublin port on Saturday.
Gardai said the man, who is in his early 20s and from Northern Ireland, was held over an unrelated outstanding court order and will appear in court on Saturday afternoon.
He is understood to be sought by Essex Police as part of their investigation, and the force confirmed officers are in touch with Irish police.
Three other people arrested over the deaths, including a 48-year-old man from Northern Ireland who was detained at Stansted Airport on Friday on suspicion of conspiracy to traffic people and manslaughter, remain in custody.
Officers had earlier arrested a couple, named locally as haulage boss Thomas Maher and his wife Joanna, both 38, of Warrington, on suspicion of 39 counts of manslaughter and people trafficking.
In Belgium, police are hunting the driver who delivered the trailer to Zeebrugge, the port it left before arriving in the UK.
It comes after the Vietnamese ambassador to the UK met with detectives as they work to identify the bodies.
Essex Police initially believed the victims were Chinese nationals.
Detective Chief Inspector Martin Pasmore told reporters on Saturday the nationality of the victims is not yet known, but the focus is now on the Vietnamese community - although “there may be other nationalities involved”.
He said he had met with Vietnamese ambassador Tran Ngoc An, who had visited the civic centre in Grays to pay tribute to the victims.
DCI Pasmore said there were “very, very few” identity documents recovered and that police will share fingerprints with Vietnamese authorities in a bid to identify the bodies.
All of the bodies have now been moved from the truck in Tilbury Docks to Broomfield Hospital in Chelmsford, for post-mortem examinations to be carried out.
Commenting on reports the lorry could have been part of a convoy of three carrying around 100 people, Mr Pasmore said investigators remain “open-minded”.
“There are lines of inquiry that are set of course to establish whether or not there are wider conspiracies involved in this and the lines of inquiry are set to identify the wider conspiracy, but I can say no more than that,” he said.
The BBC said it has been in contact with six Vietnamese families who fear their relatives are among the dead, with some having the smuggling fees repaid.
Relatives of 26-year-old Pham Tra My told the broadcaster they have not been able to contact her since she sent a text on Tuesday night saying she was suffocating.
“I am really, really sorry, Mum and Dad, my trip to a foreign land has failed,” she wrote.
“I am dying, I can’t breathe. I love you very much Mum and Dad. I am sorry, Mother.”
Her family said they had paid £30,000 for her to be smuggled into Britain, which has now been repaid.
The woman’s father, Pham Van Thin, told CNN: “The smugglers said that this was a ... safe route, that people would go by aeroplane, car... if I had known she would go by this route, I would not have let her go.”
In Vietnam, Nguyen Dinh Gia, the father of 20-year-old Nguyen Dinh Luong, feared his son was among the container victims.
He had not spoken to his son since last week when he told his father he was trying to reach the UK by joining a group in Paris.
“He often called home but I haven’t been able to reach him since the last time we talked last week,” Nguyen Dinh Gia told the Associated Press.
“I told him that he could go to anywhere he wants as long as it’s safe. He shouldn’t worry about money, I’ll take care of it.”
He explained his son left Vietnam to work in Russia in 2017 and had since passed through Ukraine, Germany and France.
It is not yet known when the victims entered the trailer, where temperatures can be as low as minus 25C if the fridge is activated, or the exact route it travelled.
Belgian officials said the trailer arrived at Zeebrugge at 2.49pm on Tuesday and left the port the same day en route to Purfleet.
The trailer arrived at Purfleet at around 12.30am on Wednesday, and was picked up by the cab, known as the tractor, which arrived from Northern Ireland via Holyhead in North Wales on Sunday.
The lorry left the port at Purfleet shortly after 1.05am before police were called to the Waterglade Industrial Park on Eastern Avenue in Grays at 1.40am.