It seems possible that, at some stage in the late eighteenth century, there was a plan that would probably have destroyed Clonmacnoise.
There exists A Map of the River Shannon from Athlone to Killaloe, Surveyed by John Killaly 1795, which contains much of interest. I do not have permission to reproduce it here, but here is the section around Clonmacnoise from the ~1840 OSI 6″ map.
I have marked on the map some of the placenames used by Killaly.
The legend reads:
B. Ford least water 4F 6J [which I take to mean 4′ 6″]
From Q to P the proposed Canal is ¾ Mile shorter than the River.
From Q to R [the proposed Canal] is ¾ Mile shorter than the River.
It seems therefore that, in 1795, someone was considering shortening the Shannon by digging one of two possible canals to cut off peninsulas along the east bank. Given the narrowness of the stretch between the esker and the river, I suspect it would have been impossible to dig either of them without destroying Clonmacnoise.
I would like to know more about the proposal and about why it was abandoned.
Incidentally, some folk prefer the spelling Clonmacnois but the Placenames Database of Ireland uses Clonmacnoise.