Thank you for that information. I wonder whether the Margaret nee Callow who was the wife of James Duggan (parents of Richard b 1792 etc) could be this one:
DUGGAN, Margaret 71 Mal 5 Feb 1829
which might go with this one:
DUGGAN, James 77 Mal 6 Mar 1839
I looked at the possibility that Val's Thomas Duggan b circa 1796 might be the son of the William Duggan and Margaret Quayle you mentioned, but they didn't marry until 1801, and their two children who show up in the IGI were born 1803 and 1806.