Jim McLean - thanks; My discovery of Hugh McWilliams was accidental - I was going through popular 19th century poetry books that had 'songs' in their title in the Linenhall Library of Belfast when I started finding 'traditional songs' in McWilliams' 1831 book. Later research proved his authorship of them.
It's not surprising that Gavin Greig didn't know about it - the Linenhall's copy is unique as is the National Library of Ireland's copy of McWilliams' 1816 volume though there is another incomplete one