Lower Shannon Defences initially included a battery of eight 24-pdr SB guns on Tarbet Island established in 1781. By 1795 indications are there were two batteries on Tarbet Island each with eight 24-pdr SB guns and three 6-pdr SB gun for land defence. The Shannon Estuary was regarded by the French as a suitable landing point for an invasion along with Bantry Bay and Galway Bay.
In 1806 Arthur Wellesley visited Tarbet Island an made some observations which were developed by General Charles Dumouriez, a French general who had defected. There was another report prepared by the earl of Hardwick in 1804, with the result that by 1808 a number of new coast defence batteries were under construction in the Shannon Estuary. The Batteries on Tarbet Island, which have now been destroyed, were of course fully operation before this date.
The defences built to defend the Shannon Estuary were principally coast defence batteries with eight 24-pdr SB guns in a fan shaped emplacement. The gorge of the battery was protected by a rectangular defensible barracks (defensible guar house), which had loopholes flanking the moat that surrounded the whole site. There was also scarp galleries providing additional flanking fire. The roof of the defensible barracks mounted two guns, although in a least one case it appears there were three guns. These guns were for land defence and were carronades or lighter calibres. This design was also used at Keelogue Battery, Upper Shannon and Rathmullan Battery at Lough Swilly.
