Mr Mahon was found guilty of the lesser charge of manslaughter after being cleared of murder last year
David Mahon has lost his appeal against the severity of the seven-year sentence he received for killing his stepson.
The 46-year-old stabbed Dean Fitzpatrick outside his apartment at Ongar Village in Clonsilla in Dublin in May 2013.
A jury cleared him of murder last year but found him guilty of the lesser charge of manslaughter.
The judges dismissed his appeal today because they felt the trial judge handed down an appropriate sentence.
Dean was 23 at the time of his death and is a brother of Amy Fitzpatrick, who has not been seen since she went missing in Spain in 2008.