Rathdown Castle was built around 1200 AD, on the site of a much earlier fort and rath.(In Irish, Rath means mound, Dun means fort).Scott, in his book, “The Stones of Bray,” suggests that the rath was undoubtedly one of the Royal residences of the first Milesian (Gaelic) King in Ireland, King Erimon. This original rath, constructed by King Erimon in 1699 BC, was a 3-sided earthen embankment, built to the north of a steep ravine, through which the Water of Rathdown still flows. (Erimon and his brother, Eber Finn, jointly ruled the country ; Erimon later became sole King of Ireland, by killing Eber Finn).
The rath has been re-aligned and reinforced over the years. Today, the remains of a double-ditch, possibly used as a moat in medieval times, can still be seen from the air.
The castle had one large square tower 34ft. by22ft, with walls 4 feet thick. A path passed through the body of the castle, and continued down the hill to the small harbour, the ‘Crykka.’ (Smal 1993).
A nearby water-mill drew water from the river to the south, the ‘Water of Rathdown.’ The depression marking the location of the mill-race is still clearly visible today. A well with a freshwater spring was situated close to the castle.
The castle was well located from a defensive point of view, with commanding views of main approaches to north and west, protected by sea to the east, and the Water of Rathdown to the south. During more peaceful times, the harbour would have provided a trading possibility.
Smal, Chris (1993) Ancient Rathdown and St. Crispin’s Cell, A uniquely historic landscape. Friends of Historic Rathdown, Greystones.