The 45-year-old is accused of murdering his stepson Dean Fitzpatrick outside his apartment at Northern Cross in Dublin in May 2013.
Dave Mahon claims his stepson Dean either walked onto the blade accidentally or deliberately impaled himself on it.
He admits holding it during an argument on the landing outside his apartment but claims it was only to illustrate a point he was trying to make.
He said Dean had pulled the knife on him earlier and he was asking him why he'd do such a thing to his “auld fella”.
On Friday, Remy Farrell, Senior Counsel for the DPP, accused him of telling lie after lie to Gardaí and described his “suicide by stepfather” theory as a fantastical suggestion.
During his closing speech, he asked why Dean would get up and run away if it was suicide. Why did Dave Mahon run away? He said it was hard to reconcile this with the evidence.
Sean Guerin, Senior Counsel for the accused, will be given an opportunity to address the jurors for a final time when the trial resumes this morning.
