The man has been named as John Niall Greenan from the Malone area of south Belfast.
The 69-year-old’s car careered into a ditch at Castlegal, Cliffoney, in Co Sligo.
The crash happened around 3 pm on Monday.
Gardai say his car was the only vehicle involved in the collision.
A post-mortem examination was expected to take today to determine the cause of the death.