Tyrn Gorthad is described as an older name for the barrow downs.
The barrow downs were also called the great downs.
The original name Tolkien used for them was Tyrn Goerthaid.
Belegoer was an earlier form of Belegaer.
The index of Unfinished Tales says that Tyrn Gorthad is Sindarin.
Ruth Noel, yes I know, says that tyrn means downs and the other
is from gurth meaning death.

All of which means, nothing much unfortunately. Best I can do
though.