This was the stronghold of the O’Molloy clan, who controlled the territory of Fir Chell until the late 16th century. The castle, originally known as Annagh Castle, would have commanded views across what are now called the Muddy Loughs. Today, only fragments remain: a rectangular tower with two circular corner towers at the north-east and north-west angles, complete with musket loops and a flat-headed window, along with possible footings of a bawn wall to the south-west.

The castle’s history reflects the turbulent changes in Irish land ownership during the plantation period. After Edward O’Molloy became a ward of Richard Chichester in 1590, the lands passed through multiple hands. In 1621, the town and lands of Killananny and Annaghmore were granted to Sampson Theobalds, a servant to the Lord Deputy of Ireland. By 1641, Terence Coghlan held these lands, but the Census of 1659 records John Dow as owner. The Atkinson family eventually acquired Kiltubbrid by the late 17th century, likely as part of the wider plantation scheme in the region.

When the Ordnance Survey visited in 1837, they found the castle already in ruins, describing it as situated on an island in the centre of a swamp with only two round towers remaining. A 100-metre causeway once connected the island fortress to dry land, providing the only access route across the marshes. This vital link was severed in 1852 when the Drainage Commissioners cut a channel through it as part of land improvement works, leaving the castle even more isolated on its watery island refuge.