Svetlana Murphy is in the company of Nojus Maculevicius, who is 18.
Gardaí said there were “serious concerns that there is an immediate and serious risk to the health or welfare of Svetlana”.
A car that the pair were travelling in was seen in the greater Belfast area on Wednesday evening
Svetlana left her home on Coast Road, Blackrock, County Louth on Monday.
She is described as being 5ft 1in with a slim build, brown hair and brown eyes.
At the time she went missing she was wearing navy tracksuit bottoms with a red and white stripe on the leg, a black zipped top and black trainers.
Mr Maculevicius is 6ft tall with a slim build, dark hair and is clean shaven.
He was last seen wearing dark grey Snickers work trousers and a black Snickers top.
He may also have been wearing a khaki green hoody, and either black boots or Adidas runners.
Gardaí said they were satisfied the car was in the greater Belfast area at 5.35 pm on Wednesday.
The public across the North of Ireland are asked to report any sightings of them or their to police immediately.
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