Yes Lucy, the lyrics (poem) can be considered to be in the public domain. The author Cicely Fox Smith died in 1954 and as far as we know, no one from her estate has ever challenged any publication of her works. The tune Daniel uses is indeed his arrangement of a traditional tune. You may find that some other existing traditional tune could fit the bill. This technique is used quite often in folk music. If you do cover it, we here at Mudcat would be interested in hearing the results! Good luck.