The collision has been cleared from the N7 inbound before Junction 4 Rathcoole.
The five-vehicle crash had completely blocked the route for a time.
Dublin Fire Brigade attended the incident and and said firefighters and paramedics worked at the scene.
One lane was later cleared and reopened to traffic before all lanes were reopened.
Traffic remains very heavy from Junction 9 Naas North past Junction 6 Castlewarden and again passing Junction 4.
Many commuters diverted off the N7 and took alternative routes towards Dublin.
As a result, traffic is still very slow on a number of surrounding routes, including the Kilteel Road into Rathcoole and the local Athgoe Road towards Newcastle, AA Roadwatch reported.