The enclosure at Formoyle was recorded in 1682 as the dwelling place of a figure called Henry Macoole and his offspring, with the Irish name given as fformalenetiene, meaning, so the account claimed, "the Chief place of the Giants." Whether or not that etymology holds up, the description captures something of the site's atmosphere: a large circular rampart, partly underground in legend, occupying a piece of ground that clearly felt significant to those who lived near it.

The earthwork itself is substantial enough to have carried real defensive weight. In May 1642, Dame Mary Browne, wife of Sir Silvester Browne of Formoyle, gave a deposition describing how men came to the gate of Fermoil demanding entry. When she refused, they came over the fence or great ditch, her words for the bank and fosse, and into the base court, which is to say the bawn, the enclosed yard that typically adjoined a fortified house in this period. That account places the enclosure in active use as a defensive perimeter during the turbulent early 1640s, when the Confederate Ireland conflict was reshaping land and loyalty across the island. The enclosure likely functioned as an outer fortification protecting both the orchards that once lay within it and the gardens immediately to the south of Formoyle House, which sits around 130 metres to the east-south-east. The 1914 Ordnance Survey six-inch map still showed a substantial bank and fosse sweeping from south through west to north-north-east, giving a sense of its original scale. What survives today is a polygonal enclosure roughly 115 metres north to south and 100 metres east to west, with a bank of earth and stone, stone-faced on its outer side, rising to about 1.5 metres at its highest. The fosse, a defensive ditch running alongside the bank, is largely infilled now, and along one stretch the bank has been levelled entirely, its former line visible only as a faint scarp in the ground.