The site’s history stretches back to around 1355, when Bishop McIlvlonagle is recorded as having built a manor house here, though the surviving structure likely dates from later periods. Throughout the centuries, this strategic position commanding views over the harbour attracted the attention of various authorities; in 1608, Sir Josias Bodley recommended installing a gun platform at what he described as ‘a castle of the Bishop of Derries’, whilst by 1622, records show that the Bishop of Raphoe possessed ‘the old castle of Calebegg’ by right of his bishopric.

The castle’s physical decline was already well underway by the mid-18th century, when Bishop Pococke noted in 1752 that only foundations remained. However, nearly a century later in 1847, enough of the structure survived for antiquarian Fagan to document its dimensions in detail; he recorded an interior measuring roughly 17 by 14 feet, with walls between 6 and 7 feet thick that still reached heights of up to 14 feet inside and 20 feet outside, despite being considerably damaged. Fagan particularly noted a small recess in the northwest gable and observed that this gable, along with the southwest side, retained the greatest height amongst what he described as an altogether mutilated ruin.

Today, visitors to Cat Castle will find precious little of this once-substantial tower house remains visible. The site, naturally defended by steep slopes on its northern and eastern sides, now preserves only a single corner section of wall standing about 3 metres high in the northeast, accompanied by grass-covered foundation outlines that mark the limits of the hillock’s summit. Despite its ruinous state, the location continues to offer commanding views of Killybegs Harbour, providing a tangible connection to the centuries when this fortification played a role in the ecclesiastical and military control of one of Ireland’s finest natural harbours.