The Dillons first arrived in 1185 and initially established themselves around Drumraney in County Westmeath, though ambitious family members later built power bases elsewhere, including a castle at Loughglynn in Roscommon during the sixteenth century. The family proved particularly adept at securing ecclesiastical positions and forging strategic marriages with prominent Pale families. Riverstown itself came into Dillon hands around 1525 when Sir James Dillon, third son of Robert Dillon of Proudstown, married Jane Rivers, heiress to the estate. By 1540, Thomas Dillon of Ryverston was serving as a juror, and a century later, the Civil Survey of 1654;6 recorded Andrew Dillon as owner of 320 acres at Riverstown, including what was already described as a ‘ruinated castle’.

The castle itself is an impressive four storey tower house, distinguished by its unusual design featuring projecting square corner towers at three of its four angles. The western tower houses a spiral staircase whilst the southern tower contains garderobes; both project southwestward from the main structure. The eastern tower, which projects southeast, appears to have served no specialised function. The original entrance, marked by a pointed archway on the northwest wall, demonstrates the defensive mindset of its builders: visitors must pass through a lobby overlooked by a murder hole before reaching another pointed doorway that opens into the ground floor chamber, a barrel vaulted space measuring approximately 5.3 by 4.5 metres.

Above the vaulted ground floor, the castle’s upper levels reveal the more comfortable aspects of medieval life. The first floor features what were once substantial windows on the northeast and southwest walls, along with a fireplace on the northwest wall; corbels in the walls supported the timber joists of the second floor above. Though the second and third floors are now largely inaccessible and shrouded in ivy, they appear to follow a similar layout with large windows that would have flooded the chambers with light. Whilst the parapet has long since crumbled away, the corner towers still rise defiantly above the main structure’s surviving walls. A later stone house was built directly against the northwest wall, cleverly incorporating the tower house’s original doorway as its own entrance, creating a fascinating architectural palimpsest that speaks to centuries of continuous habitation and adaptation.