Local tradition dates this O’Farrell stronghold to around 1400, though the punch-dressed quoin stones visible on the northeast corner suggest it may have been constructed or substantially rebuilt during the 16th century. The four-storey limestone structure, measuring approximately 11 metres east to west and 10.7 metres north to south, follows the typical defensive design of Irish tower houses with its thick walls, measuring 2.2 metres, and slight base batter on the western side.

The castle’s interior reveals a sophisticated layout despite significant collapse of the southern wall. The original entrance, centrally placed in this now-ruined wall, led to a protected lobby area flanked by a guard chamber in the southeast corner and spiral stairs in the southwest. The main ground floor chamber, though now filled with debris, originally featured single-light windows in three walls and supported a wooden floor above, with a barrel vault separating the first and second levels. An intramural passage at first-floor level, beginning in the northwest corner, likely led to a garderobe chamber; its exit chute remains visible on the exterior northern wall. Particularly interesting is an ogee-headed window on this level that was later blocked when a fireplace was inserted, one of two fireplaces added to the north wall, probably during 17th or 18th-century renovations.

The castle’s later history is well documented through the Plantation of Longford records. In 1620 or 1621, Roger Farrall received a land grant from the plantation commission that included ‘the castle, town and lands of Mornyn, with a water-mill’, marking the transition from Gaelic to English ownership. Evidence of continued occupation includes the later fireplaces and traces of well-preserved lime plaster on window embrasures, complete with impressions of wicker-centring used during construction. Wall footings extending east from the tower may represent either a later two-storey house or the remains of a bawn wall, suggesting the site continued to evolve well after its medieval origins.