Daniel McVeigh, 19, from Killavee, Portsalon, was discovered on Rathmullan beach at 4.30pm on Friday.
Gardai believe he may have fallen from a tree, a spokesman saying: “Investigations are at an early stage but it appears this may have been an accident.
“He may have sustained his injuries after falling from a tree.”
Mr McVeigh was found close to an entrance to Rathmullan House Hotel.
It’s not known if the teenager was at a beach party on Thursday night. More than 40 teenagers had been there.
The Portsalon teenager has undergone emergency surgery and his family remain at his bedside.
Meanwhile a family from Dooballagh, Letterkenny, is in mourning after the sudden death of 34-year-old father of two Stephen Malseed.
He is survived by his wife Michelle, son Shay (5) and daughter Orla (3).
ORIGINAL STORY: MYSTERY AFTER MAN DISCOVERED IN SEMI-CONSCIOUS STATE NEAR RATHMULLAN BEACHTags: