Essex Police say the man was taken into custody Friday along a highway northwest of London on suspicion of conspiracy to traffic people and conspiracy to assist unlawful immigration.
The bodies of 31 men and eight women – all from Vietnam -- were found Oct. 23 in the English town of Grays. Police said the victims were aged 15 to 44 and are believed to have paid human traffickers for their clandestine transit into England.
British police have already charged 25-year-old Maurice Robinson, a truck driver also from Northern Ireland, with 39 counts of manslaughter and conspiracy to traffic people. They say he drove the cab of the truck to Purfleet, England, where it picked up the container, which had arrived by ferry from Zeebrugge in Belgium.
In Ireland, 22-year-old Eamonn Harrison faces U.K. extradition proceedings on charges of manslaughter. Several other people have been arrested in Vietnam.
Investigators are also still searching for two brothers from Northern Ireland: Ronan Hughes, 40, and Christopher Hughes, 34, who are both wanted on suspicion of manslaughter and human trafficking, according to Essex Police.
The Associated Press contributed to this report.