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.
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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.