The incident happened on the R132 at Smithstown, Julianstown, with Gardaí now appealing for witnesses and anyone who may have relevant footage to come forward.
The first crash occurred at around 2.06am when two vehicles travelling southbound in the direction of Dublin collided. The driver of the first vehicle, also a man in his 50s, was not injured.
Following the initial collision, both vehicles were moved to the left-hand side of the road. Approximately 10 minutes later, a third vehicle struck the back of the second car.
The driver of the second vehicle had been outside his car and was standing near its rear when it was hit. He suffered serious injuries and was taken by ambulance to Our Lady of Lourdes Hospital in Drogheda, where he remains in a critical condition.
The driver of the third vehicle, a man in his 40s, was not injured in the collision.
Gardaí have preserved the scene for a forensic examination, and diversions are currently in place in the area.
Investigators are asking anyone who was travelling along the R132 around Smithstown and Julianstown between 1.20am and 2.20am to contact Gardaí with any information that may assist the investigation.
Anyone who may have dashcam footage from the area during this period is also asked to make it available to Gardaí.
