By the time Limerick Corporation built the housing estate in the 1930s, it was gone entirely. The Ordnance Survey had been recording what remained of it since the 1840s under the name "(Site of) Cromwell's Fort", though its history stretched well beyond that association.

The fort dates from the Cromwellian war in Ireland, when Henry Ireton's army arrived before Limerick in June 1651 and began laying siege to the city. The fort on the northern end of King's Island, the low-lying ground enclosed by the Shannon and its channels, was already in place when Ireton launched a pre-dawn amphibious attack on 23 June. A forward detachment reached the island and struck the fort but was driven back; several soldiers were killed or drowned attempting to retreat across the water. Limerick held out until the end of October 1651. When the fort appeared on William Webb's siege map, it was drawn as a square fortification with corner bastions, the whole surrounded by a water-filled ditch, or fosse, giving the overall plan a star shape. It was labelled simply "Ye fort in ye island." After the siege the structure fell into disuse, and when the Jacobite officer John Stevens arrived on King's Island in July 1690 following the defeat at the Boyne, he found only ruins, with part of the ground being used as a public walkway and the rest let out for grazing. The Jacobites rebuilt it rapidly; Stevens recorded in August 1690 that unarmed labourers were kept continuously at work raising a square fort with four bulwarks and connecting it to the city by a covered line of communication. By the second siege in 1691, the Williamite Colonel Michael Richards noted that it was "well frized and palisadoed" and surrounded by a counterscarp, the outward-sloping earthwork face of a defensive ditch, and a glacis beyond that. He considered it too large and too well supplied to reduce from a single battery. Limerick surrendered on 3 October 1691 and the fortifications passed to the Williamites.

Nothing of the fort survives above ground today, and the St Mary's Park estate occupies the site. For those interested in tracing what it once looked like, the printed sources are more rewarding than the landscape itself. George Story's published history of the wars, from 1693, includes both a panorama identifying "A new Irish fort" on the island and a detailed plan of the second siege showing the fort's earthworks and its double line of communication to the city. William Eyres's map of Limerick from around 1752 shows the counterscarp and glacis already fading back into agricultural land, with a few dwellings appearing near the south-west bastion. These images, reproduced in Eamon O'Flaherty's 2010 survey of Limerick maps, offer the clearest picture of what was once a substantial piece of military engineering, quietly erased by a century of decay and then by the practical demands of twentieth-century housing.