Gardai say Ms Bernadette McCreehan (18), of Riverstown, Dundalk, missing from her home since Saturday night, may have travelled across the Border.
She inquired about trains to Newry when she left her car at Dundalk railway station with the ignition keys in it on Saturday night. She had left home at 9 p.m. to attend a party in Dundalk and was reported missing by her family on Sunday after failing to return home.
She did not attend the party but was seen walking from the station. A Garda source said the matter was being treated seriously. Garda and RUC inquiries are continuing.