UPDATED at 6.05pm:
The road between Rathcormac and Watergrasshill and the M8 off-ramp have now reopened to traffic.
Emergency services responded to a serious crash outside Rathcormac this afternoon.
The single-vehicle crash involving a motorcycle occurred at around 3.30pm on the R639 near Dr Barry's Bridge.
The motorcyclist was travelling on the southern approach when the incident occurred.
The road has been closed to southbound traffic following the incident.
AA Roadwatch said: "The Rathcormac / Watergrasshill Rd (R639) is closed southbound just outside Rathcormac itself between Dr Barrys Bridge and the M8 junction as Gardaí deal with a collision.
"The off-ramp of the M8 northbound at J16 Rathcormac is also closed as a result."
The condition of the motorcyclist remains unknown.