Sixteen people have been found in a sealed trailer on a ferry sailing from France to the Republic of Ireland.
Police said the driver and owners of an articulated lorry were helping them with their inquiries.
In a statement, Ian Hampton, Stena Line's chief people & communications officer, said the 16 people are reported to be in good health.
"During the Stena Horizon's 9pm sailing from Cherbourg, France, to Rosslare in the Republic of Ireland on Wednesday November 20, one of our employees during a routine inspection discovered 16 people in a sealed trailer on the vehicle deck," he said.
"All the individuals are reported to be in good health and have been moved to a private passenger lounge on the ship where their wellbeing is the focus of our employees.
"Stena Line has alerted security and immigration officials in Rosslare so as the necessary arrangements can be made for the group on the vessel's arrival in Rosslare.
"The vessel is scheduled to arrive at approximately 3.30pm today."
Opposition party Fianna Fail's spokesman on justice, Jim O'Callaghan, has expressed his shock and demanded a full investigation into the incident.
"A full investigation must be launched into the discovery of up to 16 people being transported in the back of a lorry to Ireland," he said.
"Human trafficking is a modern form of slavery and needs to be tackled with fully resourced policing.
"Recent events show unfortunately Ireland is playing a significant part in this crime.
"These traffickers have to be thoroughly policed so vulnerable people are protected."
A Garda (police service of Republic of Ireland) statement said: "An Garda Siochana are now investigating all the circumstances of this incident.
"The articulated truck involved in this incident has been detained.
"The driver and the owners of the articulated vehicle are assisting An Garda Siochana with our inquiries."