Two men are still being questioned by Gardai after a pipe bomb was found in a car on the M50 last Thursday.
Detectives are investigating if the incident at the West Link Toll Plaza is linked to the activities of dissident republicans, the Continuity IRA.
A section of the M50 was sealed off for four hours while an Army bomb disposal squad carried out a controlled explosion of the device.
The 23-year-old driver of the vehicle, and another 56-year-old man arrested after a house search nearby, are being questioned at Clondalkin Garda Station under Section 30 of the Offences Against the State Act. They can be held for up to 72 hours without charge.
Detectives believe dissident republicans may have been supplying the device to a feuding criminal drugs gang in south Dublin. It was found in a plastic lunchbox on a baby seat in the rear seat of the car.
The Minister for Justice, Michael McDowell condemned the incident and said dissident republicans were cowardly thugs who had besmirched the name of republicans everywhere.
PA