The Central Criminal Court has heard that a 44-year-old man was drunk and angry when he stabbed his stepson in the stomach and fled the scene.
David Mahon has pleaded not guilty to murdering his partner's 23-year-old son, Dean Fitzpatrick, at an apartment at Burnell Square, Northern Cross on the Malahide Road on 26 May 2013.
The trial is expected to last two weeks.
Mr Fitzpatrick was the son of Mr Mahon's partner Audrey, the court heard.
His sister Amy went missing in Spain in 2008 the court heard, and has never been found.
Mr Mahon and Ms Fitzpatrick had been in the limelight trying to raise public interest about her disappearance.
Prosecuting counsel Remy Farrell told the jury Mr Fitzpatrick had a difficult relationship with Mr Mahon and had mental health difficulties.
On 24 May 2013, Mr Fitzpatrick interfered with Mr Mahon's bike outside a gym in Santry.
The court was told they would hear evidence of Mr Mahon trying to contact Mr Fitzpatrick afterwards to get him to come to his apartment, which he eventually did.
The prosecution case is that after Mr Fitzpatrick left, Mr Mahon followed him out and stabbed him in the stomach causing a very severe injury which killed him.
The prosecution says that Mr Mahon fled the scene trying to avoid CCTV cameras, leaving his stepson to receive help from passers by.
When he presented himself to gardaí on 26 May he told them Mr Fitzpatrick had walked into the knife, the court was told, and may have been suicidal.
Mr Farrell told the jurors they would have to contrast this explanation with the scientific evidence.
He said Mr Mahon was drunk and angry and stabbed Mr Fitzpatrick through the belly with a knife.
