Built with walls nearly seven feet thick, this square castle measures just over thirty feet from north to south and thirty-six feet from east to west. When surveyed in 1840, observers noted its exceptionally solid construction, though any outworks that once protected the structure had already vanished into the landscape.

The castle's documented history stretches back to at least 1583, when records describe it as a large, partially ruined fortress complete with a barbican on the north side and a stone-walled courtyard in considerable decay. Despite its deteriorating state, the structure retained several defensive features, including strongly built bedrooms and an iron door; clear signs of its military importance during turbulent times. That same year, the Earl and his wife Elinor granted Rathmore to Maurice Sheighan on a 99-year lease for a peppercorn rent, beginning a complex chain of ownership that would span centuries.

The castle witnessed significant historical moments, including its surrender to Carew in 1600 by followers of the Sugan Earl during the Nine Years' War. By 1615, the property had passed to James Casey, who settled it on T. Casey. The latter bequeathed it to his daughters in 1637, and they maintained ownership of both the castle and an associated mill-seat through at least 1655. These ownership records, preserved in various chancery inquisitions and civil surveys, offer a fascinating glimpse into how Irish castles passed through families and factions during a period of tremendous upheaval.