Dempsey started punching holes in the bedroom walls, which alerted a friend who entered the room to intervene.
The woman stayed in bed, too scared to leave, while Dempsey continued to kick and punch parts of the house.
He returned to the bedroom and punched her several times in the face and head.
The friend intervened again and dragged Dempsey into the lounge. The woman confronted Dempsey and told him to leave. She went into the kitchen and he followed, grabbed her, pinned her against the wall and punched her.
Dempsey eventually left and the woman reported the incident to police.
She had suffered bruising to her face and head and an abrasion to her top lip.
When found, Dempsey also had been the victim of an assault. He was arrested after being discharged from hospital and had a knife in his possession.
Defence lawyer Virginia Pearson said Dempsey disputed punching the woman more than once: "He accepts that it was quite a forceful punch.
"He lost control, he wants help and would be open and willing to engage in supervision and education to learn how to control his anger issues," Ms Pearson said.
Judge Tony Walsh said he was not prepared to grant Dempsey bail: "He has a bad history, he has convictions for violence, and what is concerning, particularly, is the number of convictions he has for possessions of knives."
Dempsey also had a history of offending while on bail.
Judge Walsh said the charges were serious and the woman was clearly frightened.
"I'm concerned there are serious issues of risk here."
He remanded Dempsey in custody until August 29 for sentencing.