Ronan Hughes, 40, from Northern Ireland, also pleaded guilty to being involved in the smuggling of migrants to Britain.
In late January, a court in Ireland allowed the extradition to the UK of Eamon Harrison, who was accused in the case of the deaths of migrants in Essex.
In October 2019, a truck with 39 bodies of migrants was found in Essex. All those killed were illegal migrants from Vietnam.
The truck driver later confessed to aiding illegal immigration.