(above)  Calf Island wall in Roaring water bay off Skibberean in Ireland.
No ferry visits this island.  I had to kayak there from Schull.

(above) - The Mourne Wall, 23 miles long.

Built around the watershed of a reservoir in Northern Ireland, between the years 1904 and 1922 to protect the water supply to Belfast, Norther Island.  Built of locally quarried granite.