¹⁹Wood-Martin, W. G. History of Sligo, County and Town: With Illustrations from Original Drawings and Plans. Hodges, Figgis, 1882. p. 122.
In later iterations of the town seal, the stones have turned into oyster shells. It is unclear if that indeed was the original intent for the drawing, which seems to have originally been a logo for a local newspaper.
https://archive.org/details/historyofsligoco00wooduoft/page/122/mode/2up?view=theater