The pedestrian was pronounced dead at the scene. No other injuries have been reported.
The local coroner has been notified, and a post-mortem examination will be arranged in due course.
The road remains closed as Garda Forensic Collision Investigators carry out a technical examination of the scene. Local diversions are currently in place.
Gardaí are appealing to anyone who witnessed the collision to come forward and assist with their investigation.
Road users who were travelling on the M20 between Junctions 3 and 2 between 5pm and 5.30pm on Sunday, and who may have camera footage, including dash-cam recordings, are asked to make it available to investigating Gardaí.
Anyone with information is asked to contact Gardaí to assist with their enquiries.
