A Mysterious Mound

On a hillock rising above Lough Bane, County Westmeath stands this small circular building. When was it originally constructed, and for what purpose? Clearly the upper section has been lost but the space inside is still roofed. A series of windows, some large others more like rifle holes, run around the walls, and there are a couple of large openings to permit access, which invalidates the idea that it might have been intended for defensive purposes. The interior also contains the remains of a fireplace. The views from this spot are spectacular, and allow 360 degree panoramas for many miles, so it may have served as a lookout point. Any ideas?