The island sits immediately east of the medieval borough of Meelick, and its single causeway approach, flanked by bog on either side, made it the kind of position that looked defensible on a map until it suddenly, catastrophically, was not.

The fortification was originally raised by Royalist-Confederate forces during the mid-seventeenth century wars, its square bastioned design, in which angular projecting points called bastions allowed defenders to cover the walls with flanking fire, reflecting the military engineering of the period. On the 25th of October 1650, Parliamentary forces attacked at dusk. A contemporary account describes the fighting at the third guard post as so fierce that soldiers came to blows with the butts of their muskets before the line finally broke. The rout was total. The Earl of Clanrickard's own wagons and tent were among the spoils, and the losses on the Confederate side were catastrophic; five hundred men, it was reported, were driven into the Shannon by a single party of Parliamentary horse and drowned together. The garrison left behind by the victors was itself surprised in July 1651, taken in its sleep and put to the sword, the governor alone spared. Sir Charles Coote retook the fort that October after two failed assaults, with 140 killed or drowned and 200 taken prisoner in the final storming. The Down Survey maps of 1655 to 1656, compiled in the immediate aftermath of the wars, record the site as 'Milcke forts' on the Galway bank and show a square bastioned structure. By the time the Ordnance Survey reached it in 1838, it was labelled simply 'Battery', its shape now trapezoidal, suggesting conversion to a later artillery position at some point in the eighteenth or early nineteenth century. The 1884 survey still showed a demi-bastion on the south side and an entrance to the east. Whatever building had stood inside was gone by then, probably cleared during the construction of the Electricity Supply Board embankment between Portumna and Meelick in the early decades of the twentieth century, a levée built to control winter flooding along the Galway side of the river. When the site was inspected in 1984, no surface trace of the fortification remained.