The Dart was delayed last night for 35 minutes after a fight broke out on one of the carriages.
The incident occurred at around 9:30pm last night at the Salthill and Monkstown station.
Gardaí were called to the scene after a customer alerted the driver to what was happening.
The Dart was delayed as they took the information of two men involved.
There were no arrests at the scene and no further complaints were made.
A spokesperson for Irish Rail said that up five people may have been involved in the incident, some of whom were under 18.
There were large crowds on the Dart returning home from the Bray Air Display that took place in Bray yesterday.
Local councillor Cormac Devlin said that the incident shows the need to set up a Garda Transport Unit to police public transport..
"I've seen too many of these types of incident on all types of public transport," said Mr. Devlin. "Security on its own isn't enough, something needs to be done."
"The Minister needs to talk to the Commissioner to get this kind of unit set up," he said.
The Dart resumed normal service after the incident at 10:30pm.
There were reports of gangs of youths drinking on the platform of the station and in the carriages, however it is unclear whether any of them were involved in the fight.