No earthwork, no trace of masonry, no suggestion that a fortification had ever occupied this small island in the River Shannon. The battery on Cromwell's Island had been quietly erased, most likely by the construction of an embankment built by the Electricity Supply Board in the early decades of the twentieth century, a levée running from Portumna to Meelick, designed to control winter flooding along the Galway bank of the river. The fort survived centuries of conflict and cartographic record only to be undone by flood management.

The structure's origins are uncertain but point toward the mid-seventeenth century wars that reshaped Ireland so violently. The island sits immediately east of Meelick, a medieval borough on the Shannon, and the Down Survey maps of 1655 to 1656, produced as part of Cromwellian land redistribution, offer the earliest visual evidence. The barony of Garricastle map in the Co. Offaly survey depicts the fort as a square bastioned work, a design typical of the period, with angled projections intended to eliminate blind spots in a fortification's defences. A label reading 'Milcke forts', meaning Meelick Forts, appears on the Co. Galway side of the river and may refer to this same structure. By the time the Ordnance Survey recorded the site in 1838, the shape had changed: the first edition six-inch map shows a trapezoidal outline named simply 'Battery', with a building inside it. The 1884 Ordnance Survey plan at 1:2500 scale adds further detail, showing a demi-bastion, a half-projecting defensive angle, on the south side and an entrance on the east. The interior building shown in 1838 had already disappeared by then. What had begun as a bastioned fort of the Cromwellian period had apparently been repurposed as a battery, an artillery emplacement, sometime in the eighteenth or early nineteenth century, its geometry adapted and a structure added within the earthen perimeter. The island retains its unambiguous name, a reminder of that mid-century violence, even as the fortification it once held has left no mark on the ground.