Constructed around 1650 to 1656, the fortification was raised directly over the remains of Dunboy Castle, an older tower house that had itself already passed through some violent chapters. Rather than clear the earlier structure away, the builders incorporated it, trimming the castle walls down to a height of ten or eleven feet, re-roofing what remained, adding an internal partition wall, and constructing an annex against the western side.

The fort itself takes the form of an irregular star, a plan typical of bastion-style military engineering that became widespread across Europe from the sixteenth century onward. The design, with its projecting angled points, was conceived to eliminate the blind spots that had made older vertical walls vulnerable to cannon fire, allowing defenders to cover every approach. At Dunboy the star has six salient angles, and the earthen bank enclosing the site runs to roughly five metres in thickness. The whole structure covers an area of about 32 by 35 metres. The eastern entrance has been completely destroyed, so the original access arrangement can no longer be read from the ground. What the site looks like today reflects the findings of an excavation carried out by Fahy, which exposed the layout as it now survives.