This information from Allmusic.com:

ACROSS THE BRIDGE WHERE ANGELS DWELL is on Van Morrison's 1982 album "Beautiful Vision."

Mae Robertson and Don Jackson recorded it on their 1997 album "All Through the Night." They attribute it to "Morrison/Murphy."

Dolores Keane calls it ACROSS THE BRIDGE on her 1999 album "Lion in a Cage." She also attributes it to Morrison & Murphy.

Hugh Murphy is also listed as co-writer of a couple of other songs that Morrison recorded: ARYAN MIST, also on "Beautiful Vision," and DWELLER ON THE THRESHOLD, which is on "Beautiful Vision" plus "Live at the Grand Opera House, Belfast," and "The Best of Van Morrison."

Hugh Murphy is also listed as an engineer on "Beautiful Vision." He (or perhaps someone else with the same name) is listed as producer, arranger, engineer, mixer, etc. on numerous other albums by other artists such as Gerry Rafferty, The Proclaimers, Richard Thompson, Linda Thompson, and Lindisfarne.

The following information from http://www.maclurg.com/SaraGrey/SaraNotes.htm.

Sara Grey recorded it on her 1994 album "Sara," where the liner notes say only: "I heard this song sung at a traditional festival near Springfield, Missouri five years ago and was told that it was an old Carolina hymn."

That means she learned it about 7 years after Morrison's album came out. Is it possible she was told wrongly? Or she misremembers what she was told?

Unless someone can find some proof that the song is older than 1982, I'm inclined to believe the Morrison/Murphy origin.