The alleged assault took place in the rear carpark of Mr Chippie and O'Boyces Corner at the Port Roundabout.
Two juvenile males arrested at the scene were yesterday released without charge.
The incident happened at approximately 10pm when there was a lot of people in the area.
While Gardai would not go into details regarding the nature of the assault, they have said it is a very sensitive matter.
Garda forensic officers carried out a full investigation as the area of the alleged assault was cordoned off throughout yesterday.
Gardai are asking anyone who was in the area at the time and may have witnessed anything to contact Letterkenny Gardai.
In a separate incident on Friday night a promotional vehicle outside Glenheaney House at the Station Roundabout had its windows smashed in.