This four-storey rectangular keep, measuring 10.5m by 8.65m, served as the FitzGerald stronghold until 1688. The castle's history is well documented; in 1605, King James I granted the lands to Donat, Earl of Thomond, though notably excluding a portion that belonged to the attainted Gerald Fitz-Richard. A fascinating glimpse into daily life comes from September 1631, when the 1st Earl of Cork recorded in his accounts that he forgave a debt of nine pounds, nine shillings and sixpence owed by Maurice FitzGerald of Gallarus, demonstrating both the castle's continued occupation and the financial struggles of its inhabitants.

The castle's architecture reveals sophisticated medieval construction techniques despite its weathered appearance. Rising from a battered base, the walls are built from split stone rubble with sandstone quoins, whilst the interior demonstrates clever use of space typical of Irish tower houses. Each floor consists of a main chamber approximately 6.15m by 4.3m, with the walls sloping inward from the second floor upwards to support the vaulted roof. This increased wall thickness wasn't wasted; the builders incorporated mural chambers in the southeast and northwest angles, along with passages within the north wall that eventually housed the stairs beginning at third-storey level. The floors were supported by beam and wall-plate construction, with rough corbels projecting from the east and west walls, whilst flagstone ceilings covered the mural passages and chambers.

Now designated as National Monument No. 65, Gallarus Castle has undergone recent conservation work including repointing of the stonework and the addition of a protective felt and concrete cap to the roof. Whilst the castle retains its impressive vaulted roof, time has taken its toll; the battlements have fallen, many window openings are damaged, and several internal doorways have lost their sandstone dressings. The ground floor entrance in the north wall is now a ragged gap, though portions of the draw bar socket survive. Despite these losses, the castle remains a remarkable example of a late medieval tower house, its thick walls still containing garderobes, wall cupboards, window seats, and the remnants of the narrow defensive loops and larger windows that once lit its chambers. The building techniques, particularly the use of flagstone lintels and the corbelling reminiscent of local dry stone traditions, connect this military structure to the vernacular building practices of the Dingle Peninsula.