Today, only fragments of this limestone structure survive; the south wall of the main tower remains standing alongside portions of the bawn (a defensive courtyard wall typical of Irish fortified houses), including a circular corner tower at the northwest. These ruins offer a glimpse into the turbulent period of early plantation Ireland, when newly arrived English and Scottish settlers built fortified homes to protect their granted lands.

The castle’s remaining architectural features tell the story of its dual purpose as residence and fortress. Two fireplace openings, one at ground level and another on the first floor, mark where the family quarters once stood, with the chimney stack still visible projecting from the external wall. The northwest corner tower, part of the original bawn defences, contains several musket loops; narrow openings through which defenders could fire upon attackers whilst remaining protected behind the thick limestone walls. This combination of domestic comfort and military practicality was typical of the period, resembling other fortified houses in the region such as Kilcolgan Castle in the same county.

Historical records suggest the castle may have been built by Peter Salconstall, who received a land grant here in 1622 during the plantation of King’s County, as Offaly was then known. The construction of such fortified houses marked a significant shift in the Irish landscape, as English and Scottish planters established their presence in areas previously controlled by Gaelic Irish families. Though time and weather have reduced Killooly Castle to ruins, these limestone fragments continue to bear witness to this transformative period in Irish history, when the need for defence shaped even the most domestic of architectural choices.