But she has key information about the slaying and is the girlfriend of one of the five-man gang involved in the robbery, gardai believe.
"This teenager -- who is only just a child in reality -- knows exactly what happened that night and she helped the killers," said a source.
The development comes as gardai have also identified the criminal who stole the getaway car.
Senior sources say "nothing of evidential value" was gleaned from forensic examinations of the stolen VW Passat.
A senior source said: "This is going to be a very very hard crime to solve; evidence is seriously lacking here. Gardai know who did it but the hard evidence is not there.
"Without the help of the PSNI, this crime might never be solved."
It is understood that the PSNI have carried out more searches as part of the investigation than gardai at this stage, and have questioned a number of gang members.
Yesterday the Herald revealed that the chief suspect for shooting Garda Donohoe is on Facebook.
He is the chief suspect for organising and participating in a previous raid on the Lordship Credit Union in 2011, in which two gangsters escaped with €62,000 in cash.
It is understood that Detective Donohoe personally questioned the suspect on a number of occasions in relation to serious crimes in the north Louth area.
The suspected murderer is facing charges in the North linked to a suspected sexual assault.