At first glance it might be mistaken for a collapsed store or outbuilding, but the corbelled chambers inside, where courses of stone are stepped inward to form a crude vault, and the peculiar arrangement of corridors tell a different story. This was once an industrial saltworks, one of the very few surviving examples of its kind in Ireland, built not to extract salt from the sea in the usual way but to refine rock-salt imported from Cheshire and boil it down into a purer product for the local fishing industry.

The operation owed its existence to a sequence of events that crossed the Irish Sea twice. In 1684, Henry Loftus had a pier built at Slade to service the harbour. The following year he rented the castle and lands to William Mansel, a man with complicated ties on both sides of his family: Mansel was his wife's brother-in-law, and had recently been implicated in the Monmouth rebellion, the failed Protestant uprising against the Catholic King James II in 1685. Whatever his politics, Mansel proved an industrialist of some ambition. He developed the saltworks to process rock-salt arriving by ship, with coal also imported through Slade to fuel both the manufactory and the nearby lighthouse. The complex was designed with four east-west corridors divided into individual corbelled chambers, probably used for storage, while a large open-roofed square chamber measuring seven metres by seven metres at the western end was likely where the brine evaporation itself took place. By the 1770s the works appear to have been in decline, and by the time the Ordnance Survey mapped the area in 1839 the site was already described as disused. Much of the eastern end was demolished later in the nineteenth century.

What survives today is fragmentary but legible. The south-west corridor remains the most complete section, running twenty metres in length with its corbelled chambers broadly intact. Fragments of piers and corbelling from the north-west and south-east corridors are also present, giving a sense of the original four-square arrangement. The site sits just south-east of the fortified house at Slade, a late medieval tower house that still stands, so the two structures together offer an unusually compressed picture of how a small harbour settlement functioned across several centuries.