Sean,
I've been having a browse through Kilgarriff and his entries are somewhat inconsistent. In the core song list (p17) he gives...
Cockles and Mussels (Yorkston, based on trad?) 1882.

In the Lyricist and Composer section it is given as Yorkston's only entry but dated 1884, likewise Forman's entry as arranger.

Then in the date list at the end it is given again under 1882.

Could the 1882 date be for the Musical Treasury publication perhaps?

Where Kilgarriff gets his 'based on trad?' query from you'd have to ask him, but it may well be he had access to the earlier version you mention.