The carpark at the back of Mr Chippie and O'Boyces Corner close to the Port Roundabout was sealed of as investigators carried out a search of the area.
The incident is believed to have taken place just after 10pm.
Two youths who were arrested were taken to Letterkenny Garda station for questioning.
Gardai and two ambulances arrived at the scene of the incident, while Gardai remained at the scene throughout this afternoon.
As part of their investigation, a number of young people within Mr Chippie were taken outside the premises and interviewed by Gardai.
Gardai have also been viewing CCTV footage which looks onto the carpark area.