The man, named locally as Sean Ferry, 19, was set upon at a house in the Annagry area at the weekend.
He remains in intensive care at Letterkenny General Hospital with what is believed to be a head injury.
Mr Ferry, a talented GAA player with Gaoth Dobhair GAA Club, is a second cousin of the Sinn Fein politician.
It is believed the victim was punched a number of times and may have fallen and banged his head.
The incident is understood to have happened in the early hours of Monday morning last.
Gardai forensic offices sealed off the house where the attack happened and carried out a full investigation on Monday.
Mr Ferry was rushed to hospital and his family have been by his bedside since.
No arrests have yet been made in connection with the vicious assault.
A Garda spokesman confirmed “Gardai in Annagry are investigating a a serious assault which occurred in the village of Annagry at approximately 4.45am on Monday morning.
“A 19 year old man received serious head injuries. No arrests have been made and investigations are ongoing.”
Locals are shocked by the attack.
“Sean is from a very well-known family in the Gaoth Dobhair area and everyone is in shock.
“Everyone is praying that he pulls through. He has a head injury and his family are by his side.
“Everyone is in shock that something like this happened,” said one local.