Louise Heffernan from Balbriggan left her school, Mount Temple on the Malahide Road, as normal on Thursday afternoon, having just completed a music exam. She was wearing a bright red jacket, knee-high boots, blue jeans, and a pink schoolbag with flowers on it. She followed her normal route home, which was to take the train from Connolly Station to her home in Balbriggan.

However, CCTV footage shows her boarding the 4.21pm northbound train at Connolly in the company of an unidentified young man. She is seen to be smiling, and does not appear to be feeling under any threat but, according to Louise's brother, Gerry Heffernan, the man is not familiar to any of the family.

"No one recognised him, none of Louise's friends knew him, " Gerry told theSunday Tribune yesterday. "He looked around 19 or 20 years old. As far as we all knew, she did not have a boyfriend, so we do not know who this person is. Obviously, though, we want to talk to him, because he must have key information about where she is."

Gardai are in the process of checking CCTV footage at every stop on the route of the 4.21pm Dart, but have yet to identify at what point Louise disembarked from the train. "We know she did not get off at the Balbriggan stop, or the Drogheda stop, " said Sgt Donal Lucey at Balbriggan garda station. "But she is a very pretty girl, very distinctive looking, and someone must have seen her on that train."

When Louise had not arrived home by 6pm, her mother immediately began to worry because it was unusual for the 16-year-old not to be home when expected. "She comes from a very happy, stable home, " said Gerry. "This is completely unlike her.

Even if she's going to be late or anything like that, she always calls. This is totally out of character, which is why we're all so worried."

The Heffernan family immediately contacted all of Louise's friends, but no one knew where she was. Her phone had been turned off. The family contacted the gardai and filed a missing-person's report that evening.

"My mother's been crying since nine o'clock on Thursday night, and my father is struggling to put on a brave face, " said Gerry. "I went out straight away on Thursday night to scour the town here, and all her friends went into the city this morning to look for her. We know she's out there somewhere, and we're just desperate for some word that she's alright."

The gardai have determined that the battery in Louise's phone is now dead. "We're still trying to trace her through her mobile, " said Sgt Lucey.

"We've already had hundreds of phone calls from people all around the country who think they've seen her. It's great to get that response, because at the moment we have no idea why this young girl would go missing. She had never done anything like this before, never done anything unusual. She is a bright girl, and a normal teenager."

The Heffernan family are also keeping a close eye on Louise's Bebo website. "She was on that every day, like all the other teenagers, " said Gerry. "But there's been no activity on the site since Thursday afternoon. We're still watching it, obviously."

Louise is described as being very slender, 5ft 6in in height with dark hair and brown eyes, although she occasionally wears blue contact lenses. "She wears quite a lot of make-up and can look older than she is, " said Gerry. "She could easily pass for being 18 or 19 years old. We just want to hear something from her, or something about her. Any information at all."

Anyone with information should contact Balbriggan garda station on 01 8020510.