Built by the Magawley family, this defensive complex once stood as a formidable rectangular enclosure measuring approximately 52 metres northeast to southwest and 45 metres northwest to southeast. The most striking features that survive today are found along the southwestern wall; a 38-metre stretch punctuated by circular flanking towers at each corner, their 4.2-metre diameter forms still pierced with gunloops that once allowed defenders to rain fire upon any would-be attackers. These towers, along with portions at the southern and northern corners, speak to the serious defensive considerations of the plantation era.

The castle itself, which once stood proudly at the centre of the bawn, now lies in ruins, though its square footprint can still be traced amongst the rubble. Historical maps from 1837 show the site in better preservation, with the castle intact and several outbuildings within the protective walls. By 1910, however, the Ordnance Survey noted the castle was already ruinous and the northeastern wall had vanished entirely. Today, the southeastern wall remains largely featureless but intact at 50 metres long, whilst the northwestern side, built against a natural slope, survives as a retaining wall with a curious angle near its middle.

The wider landscape around Carn Castle hints at a once-thriving settlement. Just 90 metres to the northwest lies Tobernakill, a holy well whose Irish name translates ominously as “well of the church” or possibly “well of the kill”, whilst archaeological evidence points to a settlement cluster about 125 metres south of the bawn. The interior of the enclosure, now uneven and used for cattle grazing, houses a modern hay barn built against the eastern wall. Old field banks visible to the south and west, along with a deep stone-lined well 50 metres southwest, complete the picture of a site that has witnessed centuries of continuous, if changing, agricultural use.