The man, who is in his 50s, was arrested yesterday morning by Gardai investigating the disappearance of Ciara Breen in 1997.
The 17-year-old vanished without a trace after she was last seen at Bachelors Walk in Dundalk in the early hours of 13 February, 1997.
She is described as being of slim build, with a sallow complexion, long dark hair and blue eyes. When last seen, Ciara was wearing a black leather jacket, blue jeans, a wine-coloured sweatshirt and a white t-shirt with an image of her own face on it.
The man arrested yesterday was held at Drogheda Garda Station for questioning under Section 4 of the Criminal Justice Act, 1984.
A Gardai spokesperson confirmed he was released this morning at approximately 7am. A file is being prepared for the Director of Public Prosecutions (DPP).
It is believed the teen, who was an only child, sneaked out of the home she shared with her mum Bernadette to meet someone. But she was never seen again.
When her mother went to check on her daughter during the night, she discovered she had gone and waited up all night for her to come home but she never saw her again.
Bernadette said: “I think she went out the front door and left the window off the latch to get back in. I sat the whole night waiting for her to come back.”
Ciara was also wearing a black leather wristwatch with a square face and green background.
Crimecall reconstructed Ciara Breen’s last movements on the night she disappeared and made a direct appeal for an anonymous letter received by Gardai on the case to come forward.
In November, Gardai said that they have interviewed two possibly witnesses in the disappearance of the teen. A spokesperson said the witnesses "have been of great assistance to the investigation team."
Investigating officers are appealing to anyone who may have information in relation to Ciara’s disappearance or who may have seen her on the night/morning of her disappearance to contact Dundalk Garda Station on 042-9388400, the Garda Confidential Line 1-800-666-111 or any Garda Station.