Nora Quoirin, who has learning difficulties, had been on a family holiday at a resort near Seremban, around an hour south of the capital Kuala Lumpur.
The teenager's parents reportedly discovered she was not in her room on Sunday morning and that the window was open.
The BBC reported that Malaysian police are using sniffer dogs in their search.
Nora, whose mother Meabh is originally from Belfast and whose father is French, is understood to have been travelling on an Irish passport.
The family are said to have been living in London for a number of years.
Family friend Catherine Cook told PA: "It's out of character for Nora to go wandering off."
She added: "I'm a mother and I burst into tears just hearing this story. I cannot imagine the hell that they are living right now. I just ask for anyone who can help them to do so as soon as possible.
"To my knowledge the French government and the Irish government are helping them and it would be great if other agencies or governments worldwide could support them."
A spokesman for Ireland's Department of Foreign Affairs said: "We are aware of the case and providing consular assistance."