This impressive earthwork consists of an oval enclosure measuring roughly 80 metres east to west and 60 metres north to south, with defensive features that vary according to the natural vulnerabilities of the site. The western side, where the fort is most exposed, boasts a substantial earthen bank standing three to four metres high, whilst the northern and southern sides appear to have been scarped. The eastern approach is protected by another earthen bank, and the entire complex is surrounded by a deep fosse with an outer bank that has been partially absorbed into later field boundaries and a laneway approaching from the south.

Within the northeastern section of the interior stands a prominent earthen mound, rising about five metres on its eastern side at a point where the ground level naturally changes. This feature has sparked considerable debate amongst archaeologists about the site's original purpose. Whilst some have suggested it might be a Norman motte and bailey castle, most scholars classify it as a hillfort, though curiously it doesn't follow the typical pattern of exploiting the hill's highest point, which actually lies just outside the enclosure to the west. The site has been tentatively identified with Ard Fothadh, a location mentioned in early Irish sources, adding another layer to its historical significance.

Local tradition maintains that McGonigle's Fort serves as the burial place of an ancient king of Tír Chonaill, the old Gaelic kingdom that once encompassed this region. From its elevated position, the fort overlooks a boggy valley that runs inland from the sea to the north, suggesting it may have controlled an important route through the landscape. Whether functioning as an Iron Age hillfort, a medieval fortification, or perhaps serving different purposes across different periods, this enigmatic earthwork remains one of Donegal's most intriguing archaeological sites, its massive banks and strategic location testament to its former importance in the region's complex history.