The flight was diverted to Cork Airport after a 24-year-old man became agitated about an hour into the two hour journey.
It is reported that he bit another passenger before being restrained.
He subsequently became unwell and then fell unconscious.
The captain of the flight declared a medical emergency and diverted the plane to Cork Airport, where it landed at around 5.40pm this evening.
It is not clear when the man died but he was pronounced dead on board the airplane.
Around 170 passengers and crew members have been taken from the aircraft and are now being interviewed by gardaí about the incident
One man, injured during the flight, has been taken to hospital.
Aer Lingus has confirmed that flight EI485 from Lisbon to Dublin was forced to divert to Cork Airport.
The airline says this was due to a medical emergency.
The man is not believed to be Irish but may have been studying in Ireland.
He was unaccompanied on the flight.
The passengers and crew members will return to Dublin by bus.
Source [RTÉ News]