The 45-year-old is accused of murdering his stepson Dean Fitzpatrick outside his apartment at Northern Cross in Dublin in May 2013.
His defence didn't call any witnesses this morning so the jury is now hearing the prosecution's closing speech.
Remy Farrell, Senior Counsel for the DPP, told them Mr. Mahon's theory that Dean was suicidal and ran on to the blade was cooked up.
