Two men are being held in police custody over separate allegations of people trafficking.
The first is due to be questioned over the death of an immigrant and illegal transportation of 34 others found inside a shipping container at Tilbury Docks, Essex, after being arrested in Northern Ireland today.
The 34-year-old, from Limavady, was arrested just after midday at Banbridge on the A1, Essex Police said.
He is held on suspicion of manslaughter and facilitating illegal entry into the UK.
Meet Singh Kapoor, 40, from Afghanistan, was found dead inside the container at Tilbury Docks on Saturday.
Members of the 34-strong group, including 13 children, survived the ordeal. They are all Afghan Sikhs from Kabul.
A separate investigation is under way after a lorry containing 15 people was discovered at a service station in Ilminster, near Taunton, by officers from Avon and Somerset Police working alongside immigration officials.
A German national has been arrested on suspicion of facilitating the unlawful entry of people into the UK in connection with that investigation.
The foreign nationals - who told police they were from Eritrea and Kashmir - included men, women and a 15-year-old boy.
An Avon and Somerset spokesman said: "We received several calls earlier this afternoon raising concerns about noises coming from the back of a lorry.
"Officers stopped the lorry at a service station on the A303 Southfields roundabout at Ilminster at about 1pm.
"They were checked over by paramedics at the scene, but there are no ongoing concerns over their health or wellbeing and they've been provided with medical support and supplies.
"The lorry, which originated in mainland Europe, has been seized as part of our investigation."
The discovery of the group at Tilbury has exposed the issue of people trafficking.
Nick Alston, the Essex police and crime commissioner, said: "This incident should highlight for us the evil of the abuse and exploitation of vulnerable people of all sorts and in many different circumstances.
"We can be sure that almost all cases of human trafficking involve exploitation by callous organised criminal gangs."
Speaking after today's arrest, Acting Detective Chief Inspector Martin Pasmore, of Essex Police, said: "This is a fast-moving investigation which has already involved a number of police forces and law enforcement agencies from across the United Kingdom and Europe.
"I am very grateful to officers from the Police Service of Northern Ireland for their assistance this morning.
"We will continue to work closely with all of our policing colleagues as we pursue our investigation to bring those responsible for this crime to justice."
The group found in the container were screaming and banging after arriving at the Essex port from Belgium.
It is thought they had travelled by lorry from Afghanistan before spending 18 hours inside the container as it travelled from Zeebrugge.
The survivors are now in the care of the Home Office after being interviewed by police and are in the process of applying for asylum in the UK.
A Home Office spokesman said: "We are continuing to provide expert support, along with the National Crime Agency, to the Essex Police investigation into the discovery of 35 people in a container at the port of Tilbury on Saturday.
"This tragic incident is a reminder of the devastating human consequences of illegal migration and we will do all we can to help bring those responsible to justice.
"All 34 are now in the process of claiming asylum in the UK and we are providing accommodation and support to those who require it while their cases are considered.
"The UK takes its international obligations extremely seriously and has a proud history of offering protection to those who need it."
The Home Office does not comment on individual cases and any decision will not be announced publicly.
The man is in the custody of PSNI, awaiting transportation to the mainland to be questioned by Essex Police.
Officers are searching his home address.
Meanwhile two men have been remanded in custody charged with a string of people trafficking and forced labour offences in Northern Ireland.
The pair, Romanian nationals, faced more than 50 charges when they appeared before Lisburn Magistrates' Court.
Ioan Lacatus, 31, whose address was given as Hanover Street, Portadown, Co Armagh and Samuil Covaci, 23, from Charles Street, also in Portadown, are each accused of 12 counts of people trafficking as well as 12 forced labour charges. They also face charges of acting as an unlicensed gang master.