An abandoned castle on the Clare bank of the River Shannon near the "Leap of Doonass" close to Castleconnell. The castle today is very impressive, perched on a rock above the River and with rock cut paths leading up to it from the water side.
The Leap of Doonass was where the power of the River Shannon met its narrowest point and as a result were powerful rapids. Here most boats had to disembark and either carry on by foot to another waiting boat or lift the boat out and carry it with them. As such this was a very strategic point on the River. The river itself was a routeway into the midlands of Ireland and this point here controlled access to it via the boat.
It is likely that the current building is an 18th century folly known as "The Turret". At this site however there was likely to have been a much older castle. There is evidence of some medieval stone work which was probably reused in this brick and mortar folly.
It is thought that there may have been a much older fort known "Fort of the cataract" at this point. Doonas was a fording point of the river and is noted in the Annals of the Four Masters in 1124 when it is called "Eas-Danainne" and it is where Turlough O'Connor crossed the Shannon on his way to plundering the MacCarthy lands as far as Foynes in Desmond.
A 16th centurty McNamara castle was later built on the same site. In 1563 it was in the hands of Conor O'Brien who was the 3rd Earl of Thomond. It was given to the Earl of Ormond around this time following rebellion by Conor O'Brien.
It was later recorded by Westropp, noting that it appears on the Down Survey maps and that it was built at the old fort of "Eas Danainn", or "the rock of Astanen".
The above comes from "The Castles and Tower-houses of Co. Clare" by Risteard Ua Croinin and Marting Breen.
Edit:
Old Photo of the Falls of Doonass
https://www.facebook.com/OldRiverShannon/photos/pcb.1977206348967699/1977206215634379/?type=3&theater