According to Colm O'Lochlainn (referred to by John Moulden a few months ago on the Ballad-L list), the Star of the County Down lyric was written by Cathal McGarvey (who also wrote the words of The devil and bailiff McGlynn) some time in the early 20th century.  The tune, of course, is much older, and first appeared in Scotland; but you can easily search the Forum for more information about it, as people ask the same questions about it on a fairly regular basis.