No trace of fortification survives above the surface, but the island's own outline may offer a quiet clue: its irregular, angular shape bears a striking resemblance to a star-shaped fort, the low-profile polygonal design developed in the sixteenth and seventeenth centuries specifically to withstand artillery fire by presenting angled bastions rather than flat walls.

An artillery battery was already in place here by 1611, making it one of the earlier coastal gun positions in the west of Ireland. By 1691, with the Williamite wars still fresh, a military report identified the island, noted as the site of an older castle, as the most suitable location for a full fort. That same year a draughtsman named Goubet drew up a plan of the proposed fortifications, though whether construction ever followed remains uncertain. The island was re-fortified as a battery in 1702, and again during the mid-eighteenth century, suggesting that successive administrations kept returning to the same strategic logic: any naval approach to Galway Bay passed within range of whatever guns were mounted there. The fortifications that had accumulated across two centuries were demolished in 1815 to make way for the lighthouse that now occupies the island.