Gardai were investigating and a spokesperson for Stena Line confirmed that the incident occurred on the 2pm sailing from Fishguard to Rosslare.
Gardai went on board after Stena Line ferry from Fishguard docked at Rosslare Europort.
A Gardai spokesperson said emergency services were alerted shortly after 5pm and went on board the ship.
“A man was arrested in connection with the investigation and is currently detained under Section 4 of the Criminal Justice Act, 1984 at a Garda station in the eastern region.
“A senior investigating officer has been appointed to lead the investigation and a family liaison officer has been assigned to provide support and information to the deceased’s family.”.
