Mr Justice Paul McDermott refused an application by lawyers for Boy A to give jurors the option of finding him guilty of the less serious charge of manslaughter.
The application was made by Senior Counsel, Patrick Gageby after the judge had finished his charge and the jury had begun deliberating.
Usually, when a judge has finished his charge, lawyers for either side can make "requisitions" - submissions on issues they believe the judge should speak to the jury again about.
But Mr Gageby said the first thing he wanted to address was the question of an alternative verdict.
He said every murder case carried the potential for a manslaughter verdict if the jurors did not find the defendant had the intent to kill or cause serious injury.
He had suggested in his closing speech to the jury that there was not "one pick of evidence" from the witness box, to suggest Boy A intended to kill Ana Kriégel or anyone else.
Mr Gageby said in this case if the jurors found Boy A was in the abandoned house but did not intend to kill or cause serious injury to Ana they would have no option but to acquit him, unless the judge gave them the option of bringing in a manslaughter verdict.
He said it was not open to the court to cut out the jurors' right to find the boy was not guilty of murder but guilty of manslaughter.
Lawyers for Boy B said that if the jury found Boy A guilty of manslaughter, then they would have to find Boy B not guilty of murder as he could not have known of a non-existent intention to kill.
Prosecuting counsel, Brendan Grehan told the judge he fundamentally disagreed with Mr Gageby and said he had not engaged at all with the facts of the case. He said the prospect of an alternative verdict had not been raised with the court or the prosecution at the appropriate time and was the "equivalent of a legal ambush".
Mr Grehan said no defence had been put forward on behalf of Boy A, other than the story he gave to gardaí when he was first questioned. The boy claimed he had not been in the house and last saw Ana in St Catherine's Park.
Mr Grehan said the court had to tailor its charge to the jury in line with the run of the case and was not obliged to put an alternative defence to the jury without an adequate evidential basis.
Mr Justice McDermott said the evidence advanced in relation to Boy A was that he was not in the house when Ana was killed.
There was also an overwhelming body of evidence that Ana's assailant inflicted very serious, catastrophic injuries, as a result of which the prosecution was asking the jury to infer that whoever assaulted her, intended to cause serious injury or to kill her.
He said he was not satisfied that the defence application had engaged with the facts of the case "in any sense".
He said Mr Gageby was asking him to invite the jury to speculate and enter into conjecture by asking him to allow them to consider manslaughter "outside the reality of the evidence".
The judge said there was a certain degree to which the application was "artificial and unrealistic" and he did not propose to recharge the jury in relation to manslaughter.
He also said it would have been appropriate, in the circumstances of this case, to canvas these matters with the court at an earlier stage.
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Watch: The route taken to Glenwood House