It occurred shortly after 9pm last night.
Gardaí are appealing for witnesses to a serious crash.
A car and a motorcycle were involved in a collision on the R418 at Fontstown Cross, between Athy and Kilcullen at around 9.20pm last night.
The motorcyclist, a man aged 20 years, was seriously injured and removed from the scene by ambulance to Tallaght University Hospital.
His condition is understood to be critical.
The occupants of the car were unharmed.
A full examination of the crash site will be conducted by Garda Forensic Collison Investigators and local diversions are currently in place.
Gardaí are appealing for witnesses to come forward.
Those with camera footage from the along the route at the time of the collision, including dash-cam footage, are asked to make it available to investigating Gardaí.
Anyone with information is asked to contact Athy Station on 059-8634210 or the Garda Confidential Line 1800 666 111.
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