The young woman was seen at about 00:30 on Sunday 12 April on Dumbarton Road near the Gallus Pub before walking along Church Street in Glasgow.
She got into a Hackney taxi on Byres Road, which drove off towards Great Western Road.
The potential witness is described as between 18 and 25 years old with long brown straight hair.
She was wearing a short pink coloured dress, a fur jacket and was carrying a large brown shoulder bag.
Det Supt Jim Kerr, From Police Scotland, said: "Our extensive review of CCTV has identified this young lady near to the Sanctuary Nightclub and the footage indicates that she may have information that could assist with our investigation.
"I must stress that she is in no way connected with the death of Karen but she may have information that could assist with the investigation.
"I would appeal directly to the woman or anyone that recognises her from the image to get in touch with the incident room on 0141 532 5443."
Karen's funeral took place in April in her native Mourneabbey, near Mallow, Co Cork.