I'm currently looking at a facsimile edition of "Songs Sung and Unsung" by Harold Boulton. In the table of contents, this song is listed as having come from Boulton's earlier 1892 publication, "Songs of the Four Nations." That book describes itself as "A Collection of Old Songs of the People of England, Scotland, Ireland and Wales, for the most part never before published with complete words and accompaniments."

While that book has "English words by Harold Boulton" for this song, Boulton names himself only as the editor of this compilation of "old songs," and therefore it's unclear whether Boulton actually thought up the words, or (as seems much more likely) only collected and provided them for the publication. The composer (musical arranger) is listed as Arthur Somervell.

There is already a 2002 posting on Mudcat with the lyrics of "The Castle of Dromore" from that book, "Songs of the Four Nations," here: http://www.mudcat.org/detail_pf.cfm?messages__Message_ID=827566