Blessed night, when first that plain. H. Bonar. [Christmas.] Published in his Hymns of Faith & Hope, 1st series, 1857, in 34 stanzas of 3 lines, and headed, " The Shepherds' Plain." In the Irish Church Hymnal, 1873, two centos are given from this poem, (1) "Blessed night, when first that plain," and (2) "Mighty King of Righteousness"; and in Mrs. Brock's Children's Hymn Book, 1881, No. 72, a cento is given as "Blessed night, when Bethlehem's plain," with "Alleluia " as a refrain. No. 73, in the same Collection and in the same metre, "Hark, what music fills the sky," is attributed to Dr Bonar in error. It forms a good companion hymn to "Blessed night, when first that plain."
-- John Julian, Dictionary of Hymnology (1907)