Gardaí are trying to establish if the truck carrying the 39 people found dead in Essex used a ferry service from an Irish port to Holyhead.
They also seeking to establish if the lorry carrying the container in which the bodies were found is the same one that entered Britain through Holyhead.
Gardaí believe the truck landed in Holyhead from Dublin Port, though confirmation of that was still being worked on by detectives in Dublin.
Garda Headquarters has yet to make a statement on the case.
A spokesman for Dublin Port said it had no comment at this time.
Containers can be carried by several trucks and drivers on long international journeys. In people-smuggling cases, migrants can be switched between vehicles en route.
However, if evidence emerges the people were in the back of a truck that transited through Ireland it would raise security and other concerns. Informed sources said it would take time to establish the movements of the impounded truck, the container, the arrested man and the 39 victims.
Although it is thought the lorry may have entered Holyhead via Dublin, Seamus Leheny, Northern Ireland policy manager with the Freight Transport Association, said there were a lot of unanswered questions.
“I understand it was a Bulgarian-registered container and the driver was from Northern Ireland, but there are a lot of things we need to find out.
“Did the same driver accompany the load since the people were put into it? The trailer may have arrived into Ireland unaccompanied, so there could have been two to three drivers involved. We just don’t know yet, and we don’t have the history of its movement.
"We're also not sure where the people entered the vehicle - was it in Ireland, or in continental Europe? If it was in Europe, there are criminal gangs that specialise in this. Lorry drivers are often threatened to take people at Calais. "
Mr Leheny said if the container had arrived in the United Kingdom from Holyhead, the only possible route was from Dublin, and that it may have arrived in Ireland from France. He also said was possible that the driver could have picked up the container without knowing what was in it.
“While there are all these unanswered questions, with freight things are easily traced, so we will be able to join the dots and see how it got there,” he said.
“You do hear of migrants hiding in the back of lorries, but I’ve never heard of anything on this scale.
“Thirty-nine people don’t get into the back of a lorry unnoticed. This has to have been organised.”
Bulgaria's foreign ministry said it could not confirm yet whether the truck had started its journey from the country. "We are still checking the information, published in the British media, and we're contacting the authorities," foreign ministry spokeswoman Tsvetana Krasteva said.
Road Haulage Association chief executive Richard Burnett said the lorry could have travelled on a ferry from Cherbourg, France, to Rosslare, Ireland, before driving to Dublin and taking another ferry to Holyhead, North Wales and then reaching Essex by road.
“It’s highly unlikely that if this vehicle has come from Europe that it’s been physically checked.
“Because of the migrant issue at Dover and Calais, you’ve got far more checks that are taking place there. You’ve got heartbeat monitors, you’ve got dogs, you’ve got CO2. Those checks are done as you drive through.
“Cherbourg, because it’s a low volume port, you probably won’t have the same security measures that they have in Coquelles, Calais, for the high number of vehicles that are stepping through there and that’s been one of the main migrant routes historically.
“If this is somebody trying to smuggle a significant number of people through then maybe Cherbourg has been picked because it’s a little easier to get through.”
Mr Burnett told PA the container appeared to be a refrigerated unit and described conditions for anyone inside as “absolutely horrendous”.
Temperatures in such units can be as low as minus 25C if frozen products are being transported, causing humans to “lose their lives pretty quickly”, he said.
“It’s going to be dark. If the fridge is running it’s going to be incredibly cold.
“The only place to go to the toilet is on board the back of the trailer.” – Additional reporting: PA
