The bomb disposal squad were called after a suspicious device was discovered in an apartment block.
The discovery was made at the Mews on Rochestown Avenue near Dun Laoghaire.
The alarm was raised around at 8.40pm when the device was found by an occupant in the apartment complex.
A garda spokesperson said a number of occupants of apartments had been evacuated. The bomb disposal squad remained at the scene late last night.
Local representative Vinny Duran Kearns said: "About a half an hour ago the guards closed the road at the junction of Johnstown Road and Rochestown Lodge."
AA Roadwatch tweeted: "There is an incident at Rochestown Ave near Johnstown Rd. Take care on approach."