The site at Cappydonnell Big consists of a subcircular raised platform, roughly 50 metres north to south and 43 metres east to west, surrounded by what remains of an earth and stone bank. Though weathered down to little more than a scarp in most places, the enclosure’s fosse still measures 4.6 metres wide with an outer bank visible from the north to east sections.

Archaeological excavations carried out between 2005 and 2006 as part of the N6 Kilbeggan to Athlone dual carriageway project revealed that this site has a far more complex history than initially thought. Originally documented as a ringfort, the excavations uncovered evidence of activity spanning several periods. At its heart lies what appears to be a Bronze Age ring barrow, complete with a stone lined cist containing disturbed cremation remains. The medieval period saw the construction of the main subrectangular enclosure, featuring a substantial ditch some 4 metres wide and 1.5 metres deep, with the excavated material forming an internal bank that created a level platform within. The north side featured a causewayed entrance with possible stone facing, suggesting this was once a high status site designed to impress, particularly given its position overlooking a nearby motte.

What makes Cappydonnell Big particularly enigmatic is the absence of any substantial structure that would justify such an impressive enclosure. The site may never have been fully completed, or as suggested by some scholars, it could have been a Gaelic Irish construction, which would explain the notable lack of medieval pottery. Later medieval activity included industrial furnaces and pits that yielded charcoal rich deposits, burnt bone, nineteen amber beads, and traces of metalworking slag. Post medieval times saw the site used for cultivation, with plough furrows and a corn drying kiln discovered in the surrounding areas. Despite extensive excavation, the site refuses to fit neatly into any single archaeological category; it shares characteristics with platform ringforts and moated sites but remains distinctly unique, continuing to pose more questions than answers about its original purpose and the people who built it.