It is easy to miss Roscommon Castle: despite the building’s immensity, it scarcely seems to impinge on the horizon. Such was obviously not the case when the castle was first constructed, since there would have been nothing of similar scale anywhere in the area. Work was initiated here in 1269 on the instructions of the Anglo-Norman knight Roger de Ufford, who served as Justiciar, or chief governor, of Ireland for Henry III. There were constant setbacks due to attacks on the site by Aedh O’Conor, King of Connacht; it appears the greater part of the castle was erected only in the years following his death in 1274. Originally much of this area was a lake, Lough Nea, and the castle stood on raised ground to the immediate south-east, surrounded by a moat fed by the lake’s waters. A stone wall stood immediately inside this ditch but the main structure was set further back and featured substantial three-storey D-shaped towers at each corner. The main entrance on the eastern side was flanked by three-storey gate houses, and there was a secondary point of access on the western front. Despite impressive fortifications, Roscommon Castle continued to be subject to attack from the native population and by the mid-14th century had passed into the hands of the O’Conors who remained in occupancy there for the next two hundred years.

In 1569 the then-O’Conor Don Diarmaid mac Cairbre surrendered Roscommon Castle to Sir Henry Sidney, Lord Deputy of Ireland. Eight years later the building and 17,000 acres were granted by the English government to Sir Nicholas Malby, who was Governor (later Lord President) of Connacht. Malby fundamentally altered the appearance of the castle by transforming it into a Renaissance fortified mansion. The northern side, which had never been of stone, was made into a three-storey domestic dwelling linked to the eastern range to form an L-shaped block: large stone mullion windows were inserted into the upper floors of the latter to admit more light than had hitherto been the case. Within the outer walls a new garden was created. On the northern side, for example, the ditch was turned into a long fish pond while formal geometric parterres were planted to the east and a grand tree-lined avenue to the south. None of these features survive.

Sir Nicholas Malby died in 1584, seemingly a disappointed man since he felt slighted by Elizabeth I who had listened to charges of corruption and violence presented by his political opponents. His estate was inherited by a son who is reported as having been slain in a battle against the Irish at Aughrim in January 1603. But even before that date Roscommon Castle had once more been subject to attack, besieged by Hugh O’Donnell for three months in 1596 and again assaulted in 1599. It changed hands on a couple of occasions during the Confederate Wars of the 1640s before being taken by a Cromwellian force in 1652. In the aftermath of this event, some of the defensive features may have been removed but further damage was apparently done to the structure during the Williamite Wars of the early 1690s. Since then it has stood in a state of decay, and today the castle’s appearance is not much different from that described almost 200 years ago by Isaac Weld in his Statistical Survey of the County of Roscommon (1832): ‘It remains merely to say a few words of the general effect of the ruins in a picturesque point of view. From several positions they make a grand and noble appearance, more particularly on the eastern side, where the towers of the portal range in a commanding line with those at the angles…In the evening, when the gleams of the setting sun are seen darting through the ruined casements and narrow loop holes, whilst the main body of the ruins remains involved in deep shade, the effect of the scene is more than usually impressive.’