It was discovered overnight, after Revenue and Customs officers stopped an Eastern European registered van.
The bomb disposal unit was calls and confirmed it was 'safe'.
The driver of the van, a woman in her 20s, was arrested and detained at Store Street Garda station in Dublin.
Gardaí say they are satisfied there was no criminal intent and that the decommissioned Russian mortar launcher appeared to be destined for a private collection in Northern Ireland.
Gardaí are continuing to investigate the importation of the firearm.
