They handed over their English and Welsh estates to their son Peter, keeping only their Irish lands, and with this move came a renewed focus on improving their castle home in County Meath. Upon their return to Ireland in 1284, they embarked on an ambitious building project: the Great Hall, constructed within the north angle of the castle ward. This wasn’t just any hall; at approximately 28.5 metres by 17 metres, it was possibly the largest medieval hall ever built in Ireland. The single-storey structure featured an impressive open interior with a roof supported by two arcades of four masonry pillars, whose rough bases can still be seen today. Below the hall’s southeast end, an undercroft served as a practical storage space and larder, originally topped with a wooden ceiling before later receiving its distinctive barrel-vaulted roof.

The hall’s design was both grand and functional. Five large window embrasures along the northeast wall, built over the earlier arrow loops of the curtain wall, would have flooded the space with light through their twin pointed arches. These windows were strategically placed to illuminate the bays between the roof pillars, whilst wall niches held bracers for what was likely a hammer-beam roof structure. The undercroft below connected to a watergate through a rock-cut passage, complete with a masonry dock platform where goods could be unloaded directly from boats on the moat. Documentary evidence shows the hall required repairs by 1326, when £20 was allocated for work on the ‘Great Hall’, with further significant repairs to what was then called the ‘Red Hall’ and the Magdalen Tower in 1367.

The fifteenth century brought further modifications to this remarkable structure. The southeast end of the hall was partitioned off, possibly to create a kitchen space, whilst the undercroft received its barrel vault and saw its doorways reconfigured for better access to the watergate, though this passage would eventually be filled in. Archaeological excavation has revealed that the undercroft remained in use well into the late seventeenth century, with finds including a coin from 1689 and clay pipe fragments from raised floor levels dating to this period. Though much altered over its long history, the Great Hall at Trim stands as testament to the ambitions of its medieval builders and the enduring importance of this Irish stronghold through the centuries.