Mr O'Toole said the accused was drunk.
He said Mr Mahon accused Dean of robbing a part of his bike and while he initially denied it, he later admitted he'd done it to annoy Mr Mahon.
Mr O'Toole said both men were agitated, but Dean said he'd return the part the following day and he got up and left.
He said Mr Mahon told him: "I'll be back in a minute" and went out on to the landing.
He said Mr Mahon wasn't gone for long, maybe one minute, and when he came back he was holding a knife.
Mr O'Toole said he got "such a fright".
He said the knife was "a large knife, a long knife".
He said Mr Mahon said to him: "You've to get me out of here. I have to go."
Mr Mahon was very agitated, so he grabbed his car keys and they left the apartment.
Mr O'Toole said they got into his taxi and he was "just driving".
He said Mr Mahon was not making any sense, and he was trying to talk to him to find out what had happened.
He said he drove on the back roads towards Swords, and recalled Mr Mahon asking him to stay off the motorways due to CCTV.
As they were coming off the M1 at Balbriggan, Mr O'Toole said Mr Mahon told him he thinks "Dean is dead" and "the knife went through him".
He said he advised Mr Mahon to go to the gardai but "Dave was not really making any sense at all".
The court heard the pair then drove back to Dublin to Mr Mahon's father's house, arriving at 3am or 4am. Mr O'Toole said Mr Mahon told his father that "Dean came at him with a knife and he ended up being stabbed".
Asked by Remy Farrell SC if Mr Mahon had said he'd stabbed him, Mr O'Toole replied "yeah".
Mr O'Toole said Mr Mahon also told his father that Dean had been sending him threatening text messages for the previous three or four days.
http://www.herald.ie/news/man-accus...irlfriend-in-neck-court-is-told-34662664.html