A house party guest who crept into a young woman's bed while she slept, rubbed his hands down her body and touched her private parts.
Charlie Potts (37) was found in bed next to the woman in a south Dublin house, where a birthday party was being held.
A court heard the victim's brother walked into the bedroom during the assault to check on his girlfriend -- who was asleep in an adjoining bed at the time -- and saw Potts straddling his sister.
The woman's own partner was downstairs at the time of the attack.
The matter was adjourned for eight weeks to allow the victim to prepare a victim impact statement.
Defence lawyer Muiriosa Regan said Potts will make a plea of mitigation on the next date.
Potts admitted before Dun Laoghaire District Court to a single charge of sexual assault in the incident on April 12 last year.
The defendant, of Urney Road, Clady, Strabane, Co Tyrone, called to the house at around 2am and was initially refused entry by the woman's brother.
Sergeant Mary Diskin had said that after some "horseplay" between the men at the door, Potts was allowed inside.
The victim was at the house and she went to bed, followed by her brother's girlfriend, who went to sleep in a separate bed in the same room.
Later, the victim awoke to find a man lying on top of her, with his arm across her waist and hand under her clothes, touching her.
Sgt Diskin said Potts put his hand down the woman's body, and touched her private parts.
The victim's brother came up to check on his own girlfriend, and saw Potts straddling his sister.
He came back up with his sister's partner and they confronted the defendant.
The court previously heard that the victim did not know Potts, but he knew some people at the party.
Potts has no previous convictions.
Judge Ann Ryan adjourned the matter for eight weeks to allow for the preparation of a victim impact statement.
Potts' lawyer said she will make a plea of mitigation on the next day.