Northern Irish lorry driver Maurice Robinson, 25, who is accused over the deaths of 39 migrants, has pleaded guilty at the Old Bailey court in London to plotting to assist illegal immigration.
Appearing by video link from Belmarsh Prison in London, Mr Robinson, from Co Armagh, did not enter a plea to 41 other charges, including 39 counts of manslaughter.
The bodies of eight females and 31 males were discovered in the trailer attached to his Scania cab in an industrial park in Essex, England, early on 23 October.
The victims were later identified as coming from various provinces of Vietnam, with the youngest being two boys aged 15.
Mr Robinson, who is known as Mo, appeared at the Old Bailey via video link for the plea hearing.
Wearing a light blue sweater and tan trousers, he spoke to confirm his identity and British nationality.
During the hearing before Mr Justice Edis, he admitted conspiracy to assist unlawful immigration between 1 May 2018 and 24 October 2019.
The charge states that he plotted with others to do "an act or series of acts which facilitated the commission of a breach of immigration law by various persons".
He also admitted acquiring criminal property - namely cash - on the same dates.
The defendant, of Laurel Drive in Craigavon in Northern Ireland, was not asked to enter pleas to other charges, including 39 counts of manslaughter.
He is charged with conspiracy to commit human trafficking offences between 1 May 2018 and 24 October 2019.
The details of that charge state that he "arranged or facilitated the travel of other persons into the UK with a view to their being exploited".
He is also charged with transferring criminal property.
The defendant was remanded into custody until a further hearing on 13 December.
Meanwhile, another man from Northern Ireland has appeared in a UK court charged with human trafficking offences in relation to the Essex lorry container case.
Lorry driver Christopher Kennedy, 23, was arrested in the early hours of Friday morning on the M40 in Beaconsfield, Buckinghamshire.
The defendant, from Darkley, Co Armagh, appeared before Chelmsford Magistrates' Court charged with conspiracy to commit a human trafficking offence.
Mr Kennedy was not asked for pleas during today's hearing.
He spoke only to give his name, age, address and to state his nationality as British.
He was remanded in custody to appear before the Old Bailey on 13 December.