The O'Briensbridge capstan

Until the opening of the Ardnacrusha hydroelectric power station in 1929, the navigation of the Shannon passed through O'briensbridge. For many years there was no towpath under the bridge and the navigation arch was the fourth one out from the Clare bank. To enable boats to safely negotiate the bridge whilst coming upstream a capstan was provided on a pier in the river and boats were hauled through on a rope. Later modifications to the bridge reduced the number of arches and also created a towpath. The surviving capstan is aligned with the towpath arch and was still in use until O'briensbridge was cut off from through navigation in 1929.

More detail about the capstans can be found on this interesting site run by Brian Goggin.