Isaac,

If you click on the third link under Related Threads at the top of this thread you will find a discussion on the origins of the song, the gist of which is that Padraic Colum rewrote an old folk song as a poem. Many poets have done this over the years. Another example is W. B. Yeats "Down by the Sally Gardens".

You will find a lot of information (some more accurate than others) by following the above links. Be aware that due to a problem that happened here a few years ago, the earlier posts appear out of sequence, so the first post you read may not be the oldest one. Its confusing until you realise this.