The accident happened at around 2.30am on the N21 at the Rathkeale Bypass, when a car collided with a parked truck.
The victim, who was a passenger in the car, died at the scene.
The driver was taken to the Mid-Western Regional Hospital.
The two men were the only occupants of the car.
A full technical examination of the road has taken place and it has been reopened.
In a separate incident, an elderly woman has been seriously injured in a crash at Oranmore, near Galway city.
The collision, between a car and a lorry, happened just after 2pm this afternoon.
There are long delays on the N6 Dublin-Galway road as a result of the collision.
Motorists are advised to avoid the area if possible.
