Dr Lynn Gibbs told gardaí she had believed there was "no hope" for herself or her daughter Ciara and planned that they both should die, the court was told.
She made the comment in one of two interviews with gardaí after Ciara was found dead by her father Gerard in the family home on November 26th, 2006.
Dr Gibbs was taken to St Luke's Hospital suffering cuts to her neck and left wrist and having overdosed on sleeping and anti-depressant pills.
She was later sent to St Patrick's Hospital and is now a secure patient at the Central Mental Hospital, the court heard.
She underwent electronic convulsive treatment, where electrodes are placed on the head and shocks sent through the brain, and could not be interviewed by gardaí until December 23rd.
The court was read two interviews conducted by gardaí, but Dr Gibbs remembered little of the night, only snatches and vague recollections.
She said her mood had been low for a number of months and she had trouble sleeping. She thought Ciara would have gone to bed that night and she got herself ready for bed.
"I would have run a bath and I remember calling Ciara and she came into my bedroom. She came into my bedroom and the bathroom. I recall pushing her under the water but I can't recall the steps in between or what I said," she told gardaí in her first interview.
She remembered taking her out of the bath and thought Ciara was dead by then.
"I remember being very low. I believed there was no hope for Ciara or myself, I planned that we both die."
She said her daughter had been suffering from anorexia nervosa since September and she believed her outlook was poor.
Dr Gobnait Carney, who was seeing Ciara about the disorder, said the 16-year-old had a body mass index of 17, where the normal was 19 to 24.
Initially she put on weight but then dropped it again and was tearful and resentful at having to see a specialist in Dublin, but no one was pointing to the gloomy, catastrophic outlook that was going on in Dr Gibbs's mind, the court heard.
London-based psychiatrist Dr Tom Fahey said Mrs Gibbs presented a picture of full-blooded depressive illness developing from September onwards and diagnosed a major depressive disorder.
"Lynn Gibbs felt that her daughter was in a state of severe suffering that wasn't going to get any better, it was going to persist, and she felt through a type of distorted reasoning that came through her illness that she should act to relieve her daughter of her suffering and take her own life at the same time," he said.
Although Dr Gibbs knew the nature and quality of her act she did not know what she was doing was wrong and in fact thought it was right and could not refrain from doing it, he said.
The best way forward was for Mrs Gibbs to remain in the Central Mental Hospital for treatment, he said.
His comments were echoed by Dr Cleo Van Veldson, who said it appeared the family had never dealt with painful incidents such as Dr Gibbs's mother's death.
Dr Gibbs had learnt to compartmentalise things, putting stressors out of her mind as she was aware of her previous depressive state.
Ciara's illness had preoccupied her, however, leaving her obsessed by it, Dr Van Velden told defence counsel Patrick Gageby SC.
"When she felt her daughter was doomed it was also a statement about herself," she said. "I don't think we'll ever fully know what it was that caused the crisis that night," she said.
The court heard that if found not guilty by reason of insanity, Dr Gibbs would remain in secure care.
Her situation would be reviewed from time to time and the director of the Central Mental Hospital would decide if and when she should ever return to the community.
The trial continues today.