A teenager charged with murdering Melanie McCarthy-McNamara has admitted writing graffiti on his cell wall, which the prosecution says bragged about killing her.

Graffiti including: "Two in the head. The b**** is dead" was found on Daniel McDonnell's cell wall while he was detained at a garda station on suspicion of her murder.

Counsel for the 19-year-old made the admission on behalf of his client today, on the third day of his trial at the Central Criminal Court.

Mr McDonnell of Brookview Lawns in Tallaght denies murdering the 16-year-old on 8 February 2012 at nearby Brookview Way.

She was shot in the head as she sat in a car with her boyfriend and his friend.

Defence counsel Patrick Marrinan SC said: "While detained at Tallaght Garda Station, the accused admits that he wrote certain matters on his cell wall, as detailed in the statement of Garda Francis Glennon and subsequently photographed by members of An Garda Siochána."

Garda Glennon later testified that he was the jailer in the station during Mr McDonnell's detention there in February 2012.

He said he noticed graffiti scratched onto the cell wall including: "Moran scummy nacker [sic]" and "Moran scum", which the prosecution claims is a reference to Ms McCarthy-McNamara's boyfriend, Christopher Moran.

"They were signed, Dano McDonnell and D McD," recalled Garda Glennon.

He said he also saw the line: "I'll do 25 on the toilet while you're crying P****. Two in."

The garda took photographs of the graffiti and these were shown to the jury.

Other officers also witnessed and photographed the graffiti.

Garda David Pidgeon said that written below "Moran scum" were the words: "Two in the head. Your b**** is dead ha ha."

The jury also heard from Dr Khalid Jabbar, who carried out the post-mortem examination, that Ms McCarthy-McNamara died of significant cranial-cerebral trauma incompatible with survival, due to a shotgun wound to her left temple.

The trial has now gone into legal argument before Mr Justice Paul Carney and the jury of six men and six women will return to court on Tuesday.