The man, who's in his 40s, had got out of his cab on the hard shoulder and was struck by a passing car.
He was airlifted to Tallaght Hospital where he remains in a critical condition this morning.
The two occupants of the car escaped unharmed.
An investigation into the crash has been launched and detectives are asking anyone who was in the area at the time to come forward.
The incident is believed to have taken place in the southbound lane of the M9 between junction 4 (Castledermot, Co. Kildare) and junction 5 (Tinryland, Co. Carlow) yesterday evening at around 7.30p.m.
