It's just got to be this one. The tune was used by an english lady for the words "Morning Has Broken", recorded by Kat Stephens, but here are the details for the original: -
Words: Mary M. Macdonald (1789-1872); translated from Gaelic to English by Lachlan Macbean in Songs and Hymns of the Gael (Edinburgh, Scotland: 1888).

Music: "Bunessan," traditional Gaelic melody (MIDI, score).


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Child in the manger, Infant of Mary,
Outcast and Stranger, Lord of all,
Child Who inherits all our transgressions,
All our demerits on Him fall.

Once the most holy Child of salvation
Gently and lowly lived below.
Now as our glorious mighty Redeemer,
See Him victorious o'er each foe.

Prophets foretold Him, Infant of wonder;
Angels behold Him on His throne.
Worthy our Savior of all our praises;
Happy forever are His own

I always thought that this Carol stood out for both the words and tune.

Try this website http://www.cyberhymnal.org/htm/c/h/childman.htm

Andy