The judge in the Graham Dwyer trial is expected to finish giving his final directions to the jury tomorrow.
Mr Justice Tony Hunt told the jury they must be satisfied beyond reasonable doubt that the architect murdered Elaine O'Hara, by stabbing her to death.
He suggested that if the jury do not conclude Graham Dwyer was the person who bought and used the so called 'secret' phones to text the childcare worker, then they should go no further in the case.
Forty-two-year-old Graham Dwyer of Kerrymount Close, Foxrock denies killing the childcare worker at Killakee in Dublin in August 2012.
Mr Justice Hunt today stressed that the jury must set aside any feelings they may have towards Graham Dwyer and adopt a clinical approach to their deliberations.
He told them proof of unusual fantasies did not constitute a crime unless the ingredients of murder were established.
He said a jury can convict on circumstantial evidence, but they must be satisfied beyond reasonable doubt that Graham Dwyer stabbed Elaine O'Hara to death for sexual gratification.