Many thanks to the learned Eoin C Bairéad for the information that the good people at logainm.ie have scanned many old maps, and other documents, and made them available here, free of charge. They include maps of counties bordering the Shannon, Murdoch Mackenzie’s 1775 chart of the Shannon Estuary (with some soundings) and Mackenzie’s Views of the West Coast of Ireland.
Note: some of the files are very large.
Irish waterways history 