Northern Irishman Ronan Hughes pleaded guilty on Friday to the manslaughter of 39 Vietnamese men, women and boys found in the back of a refrigerated truck near London last year.
Hughes appeared at the Old Bailey court following the discovery of the bodies in October.
He was one of several men accused of being part of a people-smuggling ring linked to the deaths.
Eamonn Harrison from Northern Ireland, pleaded not guilty to the same charge.
Thirty-one of the victims were men or boys, and eight were women.
The oldest was 44 and three were aged under 18, including two 15-year-old boys.
The bodies of the Vietnamese nationals were discovered on an industrial estate in Grays, Essex, shortly after the lorry arrived in Purfleet on a ferry in the early hours of October 23 last year.
An inquest heard their medical cause of death was asphyxia and hyperthermia - a lack of oxygen and overheating - in an enclosed space.
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