This unusual rectangular tower house, likely built by the O’Connors, measures 10.4 metres north to south and 8.7 metres east to west. By 1570, it had passed to Turlough O’Brien, whose family would hold it until the 1641 Irish rebellion, when Donough O’Brien lost the property to John Fitzgerald. The castle then changed hands several times before William MacNamara took ownership in 1752. Today, only the south wall survives to its original height of approximately 9.5 metres, revealing the tower once stood at least three storeys tall, built from dressed limestone blocks that deteriorate in quality above seven metres.

What makes Doonmacfelim particularly fascinating is its architectural peculiarities. Unlike most tower houses of its era, the main entrance in the north wall led directly to the ground floor hall without a porter’s lodge, whilst a circular stairwell occupied the northwest corner. The building featured vaulted ceilings over both the ground and first floors, with defensive loops set within squared and splayed embrasures on three sides. Most intriguingly, a secret chamber was concealed within the thickness of the south wall on the first floor; this hidden room, measuring 2.9 by 1 metre and standing 2 metres high, could only be accessed through a trapdoor from the second floor, requiring a precarious four metre drop. The chamber featured its own loop window, a clever spy hole disguised as the back of an aumbry in an adjacent embrasure, and remarkably, a water conduit carved through the stone with splayed ends that could be covered when not in use.

By the time antiquarian Thomas Johnson Westropp visited in 1900, he found the castle reduced to ‘a mere fragment, quite defaced’, with only its vaulted lower room still recognisable. A 1998 inspection revealed little had changed beyond the accumulation of domestic and agricultural refuse in the interior, along with evidence of burning. The site, marked on the 1842 Ordnance Survey map as ‘Doonmacfelim Castle (in ruins)’, continues to command impressive views across the hilly pastureland, with an ancient enclosure 107 metres to the northeast and a mound 144 metres to the east-southeast serving as reminders of the area’s long occupation.