The original Irish name, Dunmechanan, meaning the fort of the son of Canan, hints at something older beneath the current structure, though no archaeological confirmation of an early promontory fort has been found. What is certain is that the place was considered strategically significant well before any cannon was pointed across these waters. A castle, probably a tower house, stood here alongside a bawn wall, the latter being an enclosing wall typical of fortified Irish settlements, and both are visible on late sixteenth-century maps even as the more elaborate bastioned fort was already taking shape around them. A circular tower from that earlier castle survived long enough to acquire the name King James' tower before it disappeared entirely in the twentieth century.

The impetus for serious fortification came in 1551 to 1552, when proposals were drawn up to suppress piracy and guard the estuary approaches to Waterford and New Ross. Nothing happened until the threat of Spanish invasion forced the issue: construction began in 1587 with a rampart fifty feet long and sixteen feet wide. Within a few years there was an outer trench eight feet deep, a rampart twenty feet high, stone towers at either end, and a drawbridge. By 1591 the trench had been deepened to sixteen feet and widened to thirty-five, and four gun platforms had been established at the western tip of the promontory. The fort's subsequent history reads as a compressed account of almost every crisis in Irish and British politics between the 1640s and the twentieth century. It sheltered Protestants during the 1641 rebellion, endured a Confederate siege that ran from December 1641 to March 1642 and was only broken by the arrival of two hundred men from Bristol, then switched sides to Parliament in 1644, fell to General Thomas Preston on 19th March 1645, and surrendered to Cromwellian forces on 17th August 1650 after plague had weakened the garrison. James II embarked here for Kinsale after the Battle of the Boyne in 1690, making his way eventually to France; King William was detained at the same fort a few months later when bad weather prevented his crossing to England. During the 1798 rebellion it was the only loyalist strongpoint remaining in County Wexford, and a sortie from it towards Wexford town was turned back at Three Rock Mountain. The IRA burned it in 1921. The Irish Army occupied it during the Second World War, adding concrete pillboxes and gun platforms that sit alongside the eighteenth and nineteenth-century fabric.

Most of what a visitor sees today dates from the 1700s and 1800s, with the rampart layout preserving the logic of the earlier design. The fort is arranged in two courts. The eastern court occupies the top of the promontory and is protected by a wide flat-bottomed fosse, a defensive ditch, lined in brick and stone, with a single V-shaped bastion projecting outward at its centre; the entrance passes through this bastion, and a causeway now replaces the original drawbridge crossing. A narrow tunnel runs from the interior through the rampart to a postern gate at the ditch floor. Beneath the glacis, the sloped outer face of the defensive bank, at the southern end of the ditch, are two gun-emplacement chambers and three magazine chambers whose fields of fire cover both the fort's south wall and the estuary beyond. The western court is D-shaped and oriented to sweep the estuary from south to north-west. A garrison burial ground of similar shape lies at the northern end of the glacis, though very few headstones remain in it. Remote sensing and metal-detecting surveys of the glacis have produced evidence of ditched enclosures, pits, and industrial activity across multiple phases, suggesting the promontory was in use long before the first rampart was raised.