That a functioning police station occupies the interior of a bastioned fort is unusual enough; that the fort itself was built around a pre-existing church, absorbing an older sacred structure into a military design, makes the place stranger still.

The first fortification on the site dates to around 1601, when an irregular earthen structure was thrown up, incorporating the church already standing there. It was a pragmatic arrangement typical of the period, when speed mattered more than geometry. By around 1624, this was replaced by a more regular and more robust fortification. Cromwell is said to have raised the walls further during his Irish campaign in 1649, though the structure was recorded as being in a decayed condition by 1677. A new barrack was constructed inside the fort in 1719, beginning the long transition from purely military to administrative use. The fort is an irregular quadrilateral in plan, with two pentagonal bastions, the projecting angular platforms designed to eliminate blind spots in defensive fire, at the southern corners, and two sub-rectangular bastions at the northern corners. A demi-oval bastion projects from the centre of the north wall. The southern curtain wall, the stretch of walling connecting the southern bastions, was at some point removed and replaced. The remaining walls, battered on their outer faces (that is, sloping slightly inward as they rise, a technique that adds stability and deflects projectiles), survive to an internal height of four to five metres, with the uppermost two metres appearing to be later reconstruction. An entrance gateway was added to the east, outside the original curtain wall.

The L-shaped building along the south and west sides of the interior now serves as the modern Garda barracks, meaning public access to the fort itself is not straightforward. What is visible from the exterior, however, includes substantial stretches of the original limestone walling, and the geometry of the bastions remains legible in the plan of the site.