Once known as Caisleán a' Doirín, meaning "castle of the little oak wood", this rectangular tower measured approximately 15 metres long by 6 metres wide. When antiquarian Thomas Johnson Westropp surveyed it in the early 1900s, the gables still stood tall whilst the side walls had already collapsed; by 1840, observers noted that the gables remained perfect despite being draped in ivy, though the side walls had nearly mouldered away entirely.

The castle's footprint can still be traced through modern technology, even though nothing visible remains above ground. Digital Globe aerial photographs from 2011 to 2013 and Google Earth imagery from 2018 reveal a long building measuring 23 metres northeast to southwest by 9 metres northwest to southeast, sitting precisely where the old fortress once stood. The Ordnance Survey's detailed 25-inch maps from the 19th century depicted it as an elongated rectangular structure, carefully annotated as "Derreen Castle (in ruins)".

Today, the site sits in gently undulating pasture land, with a tract of forestry to the north and another modern dwelling about 40 metres to the west. While the physical castle has vanished, its memory persists in local knowledge and historical records, a reminder of the many tower houses that once dotted the Irish countryside, providing both defence and status for the families who built them during the turbulent medieval period.