Gardaí say they have recovered a loaded firearm as part of an investigation into organised crime.
A man in his 20s was arrested at around 3.15pm at Connolly train station.
Gardaí boarded the 15:20 train from Connolly to Belfast and arrested the suspect.
The firearm was also recovered during the operation, which involved the Special Crime Task Force, the National Drugs and Organised Crime Bureau, and the Armed Support Unit.
The man was brought to Store Street Garda Station, where he is currently detained.
A woman in her 20s was later arrested at Dublin Airport as part of follow-up searches.
She is also being held at Store Street Garda Station.
Gardaí say their investigation is ongoing.