According to http://www.jump.net/~pwworth/alph-am.html (plus some other sources):

"The Angel's Serenade", which John McCormick recorded in 1914, was an English translation from the Italian "La Leggenda Valacca", composed in 1867 by Gaetano Braga (1829-1907) with lyrics by M. M. Marcello. The Italian lyrics begin "O quali mi risvegliano".

AMG/All Music Guide lists 3 albums that contain McCormack's recording:

"Ireland My Ireland" Pro Arte CD CDD-476, 1991.
"More Rarities" Rego Irish CD 3960, 1996.
"Minstrel Boy" ASV/Living Era CD 5224, 1997.

The "Minstrel Boy" album is listed on CDNOW with a sound sample. However, the recording quality is so poor it was very difficult to make out any words. Here's the best I could do:

"Whose songs are those that waken me,
Sweet ... and tender
If thou not be ... floating by
Naught can such sounds..."

The tune was used as a theme for the "Amos 'n' Andy" TV show. The recording made for that purpose featured a choir vocalizing "Ah-ah-ah..."--no words. It is included in the album "Television's Greatest Hits, Vol. 4" 1996.