Maybe my memory is failing, but I can't recall seeing this as the origin of the name before;

THE ORIGIN OF THE NAME.
A few weeks before this split took place the society at Plymouth attempted to impose its views, concerning the pastorate, upon the friends in Ireland, but the society in Cork retorted, "We will not be overruled by these Plymouth Brethren." The public at once caught the spirit of this designation, and " Plymouth Brethren " has been persistently applied to all the parties and factions of that persuasion ever since although the Brethren themselves protest against the appellation

 Same story is told in this pamphlet; https://archive.org/details/governmentofking00port/page/n5