Darci Day, who is from the US state of Maine, testified that Mr Dwyer had told her that his fantasy was to stab a woman to death during sex.
Ms Day was giving evidence via live television link from the US on the 29th day of the architect’s murder trial.
Mr Dwyer, 42, is charged with murdering Dublin woman Elaine O’Hara at Killakee, Rathfarnham, Dublin, on August 22, 2012.
The Cork-born father-of-three of Kerrymount Close, Foxrock, Dublin, has pleaded not guilty to murdering the 36-year-old childcare worker on that date.
Ms Day told the trial she had difficulties from an early age. Her parents had divorced, and her father had relationship and alcohol problems and a couple of different wives, and her mother had a boyfriend with issues.
She said she started to go online and talk to people with similar thoughts and fantasies, recalling that one of the sites was called darkfetishnet.com. She was asked if the name, Graham Dwyer, meant anything to her.
“It does,” she replied. “I can’t remember what website I found him on, but we talked a lot through Gmail.”
She was asked about the fantasies they discussed. “A lot of it was rape and murder fantasies,” she said. She said they used to exchange photographs and YouTube videos of “either throat cutting or strangulation”.
She said she didn’t know much about Mr Dwyer, but knew that he had a wife and liked planes.
“He did mention Elaine O’Hara,” she said. “I knew they had an intimate relationship and he told me she was similar to me and was suicidal.”
She was asked if he had said what they did. “He said he used to cut her … in the stomach area and stuff,” she replied. “That it was mutual and sexual.”
She was asked if Mr Dwyer and Ms O’Hara were doing anything else at the time.
“Bondage-type stuff,” she replied, confirming this involved tying her up.
She was asked if the accused had mentioned Ms O’Hara asking him to do anything.
“I believe she had asked him to kill her in the past,” she replied.
When asked what Mr Dwyer had said about that idea, Ms Day said that she didn’t remember much about it. “His fantasy was basically wanting to stab a woman to death during sex,” she said, when asked if Mr Dwyer had expressed a specific fantasy.
She was asked if he had discussed fantasies about Ms O’Hara. “Yes,” she said, pausing a number of times before continuing. “That he basically wanted to go after her and, if she wanted to, he wanted to kill her and come after me,” she said.
Under cross-examination by Remy Farrell, defending, she agreed she had been diagnosed with ADD, PTSD, and learning difficulties, but said she was currently not on any medication.
She said that things had changed amazingly for her since her contact with Mr Dwyer in 2011.
After a short break, Mr Farrell asked about a music video posted on her Facebook page recently in which a woman was having sex with a corpse.
“One where a friend tagged me in,” she said.
“You described it as something hard to look away from,” noted Mr Farrell.
The trial continues.