The 16 people were discovered on Thursday morning during a “routine inspection” of the vehicle deck, according to Stena Line.
It is understood that the group was found after banging was heard on the side of the trailer shortly before 10:00 on Thursday.
All 16 people were reported to be “in good health” and have been taken to a private passenger lounge, according to a Stena Line statement.
They were quickly released from the trailer and brought to a passenger lounge.
The 16 are believed to be all men, aged approximately between 20 and 40 years old.
A spokeswoman for Iarnród Éireann reports that Stena Line had informed port authorities about the discovery of several people in a lorry on Thursday morning.
She said Iarnród Éireann will “co-operate fully with the Garda and immigration authorities” and facilitate them at the port in any way they can”.