The 45-year-old, who's married to Dean's mother Audrey, stabbed him outside his apartment in north Dublin in May 2013.
On May 25th 2013, passersby found Dean Fitzpatrick lying on the ground outside his stepfather's apartment complex at Northern Cross.
He later died from a 14cm deep stab wound that cut through his main blood vessel.
Dean and his stepfather argued inside the apartment a short time before the stabbing. The court heard it was over a stolen water bottle.
Mr. Mahon followed him out to the landing where he pulled from a pocket a knife he claimed he'd taken from his stepson.
He said Dean then walked onto it and even suggested he may have wanted to. Earlier this month, he was cleared of murder but convicted of manslaughter.
The production of the knife and its subsequent disposal were considered as aggravating factors by Ms. Justice Margaret Heneghan while his remorse and co-operation with Gardai were taken into account in his mitigation.
The 45-year-old has been jailed for seven years.
