The N7 Limerick Dublin road has fully re-opened following a serious crash which caused major delays during the evening rush hour.
It happened after Gardai intercepted a car on the Naas Road, while investigating reports of a robbery in nearby Athy.
Three men in their teens have been arrested, and two people have been injured.
Gardaí were alerted to an attempted robbery in Athy at around 3pm this afternoon, after intruders disturbed by the return of an owner fled the scene by car.
In a follow-up operation, a Garda anti-burglary unit intercepted a car on the N7 Naas Road - which stopped briefly before accelerating away in the direction of Dublin, colliding moments later with a second car at the Johnstown junction.
One of the occupants of the first car, a man in his teens, was injured in the collision and taken by Ambulance to Tallaght Hospital with serious injuries.
Three other youths, all male and understood to be in their teens, were arrested during follow up searches near the crash site a short time later - and taken to Naas Garda Station for questioning, under section 4 of the Criminal Justice Act.
The driver and sole occupant of the second car was taken to Naas Hospital with minor injuries.
Investigations are ongoing and GSOC is also reviewing the incident.