The single vehicle collision happened on the M7 at Junction 22 Roscrea and a youth was pronounced dead at the scene and later taken to Limerick Regional Hospital where a post mortem is due to take place later today.
It is understood that there were three other passengers in the car at the time of the collision, two women aged 20 and 19 years and another 18 year old male.
Passengers were taken to Portlaoise Hospital, but have since been released.
The M7 will remain closed for most of this morning to facilitate a forensic collision examination and diversions are in place.
Motorists travelling northbound are asked to exit at junction 23 for Moneygall and can rejoin the M7 again, using junction 21 at Borris in Ossory.
Go ndéana Dia trócaire ar a anam dílis.