Here's how many hits I get on Google:
"Rares Hill" - 719 hits
"Reres Hill" - 226 hits
"Rare's Hill" - 144 hits
"Rarey's Hill" - 6 hits
"Rere's Hill" - 5 hits
"Rarie's Hill" - 2 hits
Every other spelling I can think of – 0 hits

(The above are not necessarily references to a song.)

Here are the recordings I can find at allmusic:

RARE'S HILL, sung by Mary Black on "Mary Black," Gifthorse CD #10004, 1982.
RARE'S HILL, sung by Greenhouse on "I Lie Awake," Jackalope CD #1247, 2000.
RERES HILL, sung by Old Blind Dogs on "Fit?", Green Linnet, CD #1214, 2001.

Other references found with Google:
RERES HILL, sung by Susie Allan on "Tipsy Courting," Ardo, 1998.
THE BACK O' RERES HILL, 3 versions from various sources published in the book "Songs and Ballads of Dundee," by Nigel Gatherer.