It's a hymn:  The Son of God goes forth to war,  written by Reginald Heber in 1812.  There are two tunes at the Cyber Hymnal (see link), neither of which is The Moírin, to which Thomas Moore set his Minstrel Boy.  Certainly, though, the hymn was sung to that tune in the film, and is, I gather, often sung to it in religious services.  Whether or not Heber knew the tune I have no idea.

Incidentally, there has been a lot of discussion here in the past about Moore's song, (and his lyric has been posted many times) though you need to search for minstrel, not "minstral, to find most of it!