That width, roughly a mile from Battery Point to the County Clare shore to the north-west, tells you something about the ambition of whoever ordered it built, and about how seriously the river was taken as a corridor worth defending.

According to Paul Kerrigan's 1995 study of Irish coastal defences, the battery was under construction between 1794 and 1795. It was designed to mount six 24-pounder cannon, heavy artillery pieces capable of sending an iron ball well over a kilometre, more than sufficient to threaten any vessel attempting to pass through the narrows of the lower Shannon without permission. The timing places it squarely within the period of Revolutionary France's wars, when Britain and Ireland were anxious about invasion from the Atlantic and river estuaries like the Shannon were identified as vulnerable entry points into the interior. An earthwork battery of this kind was a practical and relatively quick solution: constructed from compacted earth rather than stone or brick, it could absorb cannon fire better than masonry and could be raised without the delays of quarrying and construction.

Foynes Island sits in the Shannon Estuary and is accessible, though the battery site itself is now thoroughly reclaimed by dense vegetation. Anyone hoping to locate the earthworks should be prepared for the possibility that little is visible above ground. The point marked on older maps as Battery Point gives a geographical anchor, and the view across the river from that position, a full mile of open water towards Clare, makes the strategic logic of the placement immediately legible, even if the structure itself has largely disappeared back into the landscape.