The location sits between a small stream to the east and south, with a crannog visible just 50 metres offshore to the west. Brian Mac Aodha Óg Mac Mahon, the last elected chief of the Mac Mahons who died in 1622, made this his power base. After 1614, he was said to have retired to ‘a faire new House’ here, which may well have been the castle itself.

Today, visitors can still trace the defensive earthworks that once protected this promontory. A substantial grass-covered fosse, measuring 12 metres wide at the top and narrowing to 4 metres at its base, curves from west-north-west through north to north-north-east, effectively cutting off the promontory from the mainland. This created an oval enclosure roughly 65 metres by 30 metres, with scarped sides rising 1.8 to 2.4 metres high at the western and eastern edges. Though no structures remain visible above ground and there’s no evidence of a causeway across the fosse, the earthworks clearly define what was once a formidable defensive position.

The site appears on the 1907 Ordnance Survey map as the ‘site of Roosky Castle’, and whilst the Down Survey of 1656-8 marks Ruskill (Roosky) in Killkeevan parish, it doesn’t show any structure. Local folk tradition preserved in the Irish Folklore Commission archives confirms this was indeed a McMahon castle, maintaining the memory of this once-important seat of power in County Monaghan’s contested borderlands.