The driver, a man also aged 20 years, was taken by ambulance to the Regional Hospital Mullingar. His injuries are not life threatening.
The parked car was unoccupied at the time.
Garda Forensic Collision Investigators are examining the scene and the road at the crash site is currently closed.
Local traffic diversions are in place. The local coroner has been notified.
Investigating gardaí are appealing for witnesses to come forward. They are also asking motorists video footage, including dash cam, who were travelling on the R148 in either direction at the time of the collision, to make it available to them.
Anyone with information is asked to contact Mullingar Garda Station (044) 938 4000 or the Garda Confidential Line on 1800 666 111.