Blackhall Castle, as it was known, was positioned strategically within the medieval town, just 50 metres east of the Dominican Abbey and 140 metres southeast of another castle site. First documented in 1632, the structure gained particular significance during the Commonwealth period when it was described in 1653 as ‘one castle and hall with a garden plot called the black hall’, then occupied by Lieutenant Robert Harrysson and repurposed as the town’s storehouse.

By 1667, when Sir Arthur Forbes received confirmation of his land grant under the Acts of Settlement and Explanation, the castle had evolved into a substantial property comprising five house rooms with adjacent closes and parks, including ten acres of meadow. Richards’ 1691 map of Mullingar provides one of the most detailed visual records of the castle, depicting it as a rectangular building aligned north to south and intriguingly annotated as a ‘store of bread’, suggesting its continued use for provisioning the town. The map shows the castle at the western end of Blackhall Lane, close to what is now the junction with Grove Street.

Like many of Ireland’s urban castles, Blackhall Castle was likely one of several fortified townhouses similar to those found in Ardee and Carlingford, County Louth. These structures were still common enough in 1682 for Piers to note that Mullingar’s ‘ancient buildings here were old fashionable castles, some of which remain yet, and some are demolished, and better or at least more commodious houses are built in their room’. Blackhall Castle met its end sometime before 1837, when the Ordnance Survey map marked only its former location as ‘Site of Castle’. Today, a car park occupies the space where this once important civic and military structure stood, leaving only archival records and old maps to tell its story.