William Ó Fearghail (O'Farrell) built the original fortification here in 1405, positioning it strategically atop the hill where earlier Norman earthworks already stood. The castle's early years were marked by conflict; English forces seized it in 1420, but their occupation was brief. Rather than risk the English returning to use his own fortress against him, O'Farrell made the bold decision to demolish it himself, preferring ruins to an enemy stronghold.

Despite its destruction, the site remained significant enough that new structures were likely built there, as various grants and documents from the late 16th and early 17th centuries continue to reference a castle at Granard. An intriguing map from the early 1600s, now housed in the British Library, shows a castle standing proudly on the hilltop, occupying the same spot as the earlier motte and bailey. This suggests that the O'Farrells or subsequent owners reconstructed some form of fortification on the ancient defensive site, layering centuries of military architecture in one location.

Today, visitors to Moatfield can still glimpse fragments of this storied past. At the southeastern base of the motte, a short section of castle walling survives, standing just two to three courses high beneath its covering of grass. These modest remains gained new significance in the 19th century when John O'Donovan recorded a fascinating local account: about fifty years before his time, excavators had opened the mound and discovered intact arched vaults within, constructed from finely cut square stones bound with quality lime and sand mortar. Though these vaults are no longer visible, their reported discovery hints at the substantial medieval structures that once crowned this commanding hilltop position.