It's believed Kirsty Salmon - who has been living in the Mullingar area of Co Westmeath in the Republic - may have travelled to Northern Ireland.
She is known to frequent Downpatrick, Belfast and Lisburn, police said.
Kirsty is described as around 5' 4" tall of slim build with dark hair and pale skin.
She was last seen on August 7 and was wearing black leggings, a white tank top and a black zip-up top.
Police have asked anyone with information to contact them on 101.
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