Two Dublin teenagers have pleaded not guilty to violent disorder during an incident that resulted in a teenage girl being knocked "headfirst" under a Dart train.

CCTV footage emerged of a 17-year-old girl at Howth Junction station falling between the platform and a stationary train on 1 April last year. She was helped back onto the platform by her friends and staff.

Gardaí charged three boys, aged 17, with violent disorder. One of them had an additional charge for assault causing harm to the girl.

Last month at the Dublin Children's Court, Judge Paul Kelly held that two of the boys had a peripheral role and he accepted jurisdiction in their cases.

However, he said, the third boy, with the extra charge, lunged at the girl with "devastating consequences", and she went headfirst under the train.

He held that this teen should be tried in the circuit court, which has tougher sentencing powers.

The case was back before Judge Kelly following a four-week adjournment.

He acceded to State solicitor Anna Bridgeman's request to grant more time to complete a book of evidence for the boy facing more serious allegations. He was ordered to come back in two weeks.

His two co-defendants, who had lesser roles, pleaded not guilty and will face their hearing in May at the Children's Court.

Earlier, Judge Kelly watched the video footage and heard an outline of the allegations from Garda Kevin O'Boyle.

He told the court 10 to 15 youths "shouting and roaring" caused a disturbance on the train.

Security officers were preventing them from re-entering the train.

Six girls ran onto the platform to catch the Dart.

One of the boys swung his foot at a girl from his bike, making contact with her face.

One person with a bicycle was accused of using the handlebars to lunge at a second girl striking her knee, and "as a result, she fell down off the platform headfirst".

An OCS security man raised the alarm with the train driver and pulled the girl from under the Dart train back onto the platform.

She had a cut knee and a bad cut and bruise on her back and was visibly distressed and in shock. The court heard she was in Leaving Cert year at the time and already suffering from anxiety.

The garda said she is on medication and "going through a difficult time".

The court heard the group of teenagers were not known to the victim.

Counsel said there was a high degree of recklessness, and the defence described his actions as a "very stupid move".

They have been ordered to obey bail conditions.