An inquest into the death of 22-year-old Shane Clancy has returned an open verdict.
Mr Clancy stabbed another man, 22-year-old Sebastian Creane, to death in Bray last August, before turning the knife on himself.
His mother told the inquest she believed his actions were the result of his taking prescribed anti-depressants.
The jury heard evidence that Mr Clancy was known as a kind and charitable young man.
But after he broke up with girlfriend Jennifer Hannigan in April last year, he changed and became depressed.
According to Ms Hannigan he became jealous of her new partner, Mr Creane, and began behaving strangely.
In July he was prescribed anti-depressant Cipramol, and according to his mother quickly became agitated, and tried to take his own life.
Earlier, the inquest heard from Detective Inspector Frank Keneghan from Bray Garda Station who headed up the investigation into Mr Creane and Mr Clancy's deaths.
He outlined the series of events leading up to the deaths, which began with a night out involving Shane Clancy and Sebastian Creane, who shared mutual friends.
He said after the night out, Mr Clancy and Mr Creane arrived at Mr Creane's house at Cuala Grove in Bray at around 3.20am.
After a period, Mr Clancy, from Dalkey, Co Dublin, left and went to Dunnes Stores in Cornelscourt and bought a block of five knives.
In the meantime, Ms Hannigan, who was Mr Creane's girlfriend and an ex-girlfriend of Mr Clancy, arrived at Mr Creane's house.
She then got a text message from Shane Clancy saying he had done something stupid and was going to die.
She rang him, and while on the phone saw him outside the house, holding his chest.
Mr Creane went outside to help and was stabbed once in the chest by Mr Clancy.
Mr Creane then managed to go upstairs to his brother Dylan, who was in bed with his girlfriend and told him he had been stabbed. Sebastian Creane then went into his parent’s room where he was later found dead.
Mr Clancy then forced his way into the sitting room where Ms Hannigan was and attacked her, stabbing her in the back with the same knife.
During this, the handle broke and the knife remained in her back. Mr Clancy then went to the kitchen and got another knife which he used to stab Dylan Creane, who had come downstairs, nine times.
Mr Clancy then went into the garden and stabbed himself 19 times. His body was later found in a crouching position in a shrubbery in a pool of blood, with the knife next to him.
Det Insp Keneghan said on the basis of the evidence, it was decided that the gardaí did not need to seek anyone else in relation to the case.
In a moving deposition which she delivered to the court herself, Shane Clancy's mother Leonie said she firmly believes his actions were the result of taking prescribed anti-depressants.
She said she could not do much now to undo what had been done, but she could highlight the issue.
She also said she believed an investigation was required.
Ms Fennell described her son as a kind, caring, charming and charitable young man.
She apologised to the Creane family and to Jennifer Hannigan for what her son had done and said she could not justify what he had done.
But she said it was not in his make-up to do what he did.
She described how he had become depressed after splitting up with Jennifer Hannigan, and how that depression had worsened over the course of months.
She said he was prescribed a course of the anti-depressant Cipramil and within days became very agitated and tried to take his own life by overdosing on them.
He then went back to another doctor and explained what he had done and how he was feeling.
She said she was surprised that he came out with a further course of the same anti-depressant at a lower dosage.
A little over a week later he carried out the murder suicide.
Speaking after the verdict was delivered, clinical psychiatrist Prof Patricia Casey, who was at the inquest representing the College of Psychiatry of Ireland, read a statement in which she expressed the college's disappointment at the Coroner's decision to not allow it give evidence.
She said there were aspects of the evidence which the college took issue with and it would elaborate further on the details in the coming days.