Gardaí have seized a loaded firearm during an operation at Connolly Station in Dublin city centre.
His girlfriend was also lifted by gardai.
A woman in her 20s was later arrested at Dublin Airport during follow up searches.
A spokesman for Iarnród Éireann confirmed a man was removed from the train just before it departed and that the service was delayed for about 15 minutes.
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It is understood the man aged in his 20s was arrested after Gardai were alerted to the incident which occurred on the 3.20pm train to Belfast.
The gun was recovered and the man was brought to Store Street Garda Station where he is now detained under Section 30 of the Offences against the State Act, 1939.
"The investigation is ongoing and further updates will follow".
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