Gardaí are investigating what Iarnród Éireann has described as the single biggest vandal attack on one of its trains.

The transport company said a gang of people sprayed graffiti all over a train carriage in Clongriffin last night before threatening passengers with pieces of wood.

The group held back the carriage doors, and blocked the train from leaving the station at about 10:40pm last night.

Iarnród Éireann spokesperson Barry Kenny said it was a deliberate and calculated attack.

"While vandalising the train they had been blocking the doors from closing and stopping the train from leaving," he said.

"When customers challenged them about this, pieces of wood they had with them they threatened the passengers with those.

"While we battle with graffiti all the time, we have never had anything of this scale", he said.

Mr Kenny said there were more than a dozen people involved in the attack, which he described as "very serious."

"Certainly more than a dozen graffiti vandals basically deliberately coordinated an attack on the Dart," he said.

"[They] descended on it, blocked it from departing and when customers saw this and confronted them, they then threatened some customers with wood which they had with them as well - so very serious."

He said Gardaí will be reviewing the incident and increased security measures will now be put in place.

Dublin Senator Aodhán Ó Ríordáin said the attack is "no surprise to the local community who have been reporting serious anti-social behaviour at Northside stations for months." 

"It is now time to consider Garda policing on DART services and stations for a limited period of time to reassure passengers and to tackle this issue seriously", he said.

Reporting from Brendan O'Loughlin