Built in the early 16th century by either the McGrath or McCreagh family, this defensive structure passed into the hands of Garret Fitzgerald of Dromana by 1640, when contemporary records described it as a small but defendable castle. The rectangular building measures approximately 10 metres north to south and 8 metres east to west, its thick walls creating a compact interior space of about 5.8 by 3.3 metres.

The castle’s original defensive features tell the story of its turbulent past. The pointed doorway, positioned towards the western end of the north wall, once led visitors into a lobby with mural stairs, though these are now preserved within a wall cupboard. A spiral staircase, beginning at the first floor’s northeast corner, provided access to the upper levels. When antiquarian John O’Donovan visited around 1840, he noted the building stood three storeys tall with finely dressed corner stones, but today it has been reduced to two storeys with an attic space. The structure retains its characteristic external base batter; a sloping wall design that helped deflect projectiles and strengthen the building’s foundation.

Modern renovations have transformed this medieval stronghold into a private dwelling, though much of its original character has been lost in the process. The vaulted ceilings that once supported the upper floors have disappeared, and none of the original windows survive. Despite these changes, Mountain Castle remains an intriguing example of the tower houses that once dotted the Irish landscape, built by local families seeking both comfort and security in an uncertain age. Its position 60 metres above the Finisk River would have provided its inhabitants with commanding views of the surrounding countryside, allowing them to spot approaching visitors, whether friend or foe, long before they reached the castle walls.