It happened yesterday between the Castledermot and Carlow junctions.
Gardai are renewing their appeal for information on yesterday's M9 collision, in which a man died.
The crash, involving three vehicles, happened just after 7am on Wednesday morning, southbound, between the Castledermot and Carlow exits.
Two people were airlifted to hospital from the scene.
An 80 year old man, the driver of one of the cars, died in hospital yesterday afternoon.
A man in his 30s who was another driver was airlifted to hospital in a serious condition, while a woman in her 30s who was driving the third car sustained non-life threatening injuries.