The south-west enclosure sits in the middle of this sequence, a secondary addition squeezed between an earlier northern enclosure and a later, more elaborate south-eastern one. Its presence was confirmed through excavation, and its outline, once recovered, revealed two probable entrances: a causeway or bridge on the south side, and a further causeway on the east side. The whole arrangement speaks less to a single moment of construction than to a place that kept being rethought, expanded, and reinforced over time.

The fuller picture comes from the Draft Conservation Plan for Carrickmines Castle, compiled by Jason Bolton in 2015 and drawing on detailed excavation reports. According to that document, the three enclosures, north, south-west, and south-east, were built within the space of a few generations by the Lissebon family. The northern enclosure is considered the earliest, possibly a moated site, that is, a domestic or defensive complex surrounded by a broad water-filled ditch, and it contained at least one substantial stone building. The south-west enclosure followed, and the south-eastern one came last, with the most complex defences of the three: a trivallate arrangement, meaning three concentric lines of defence, with the innermost and central ditch cut directly into bedrock and sections of the outer works revetted, or faced, in granite. The foundations of a rectangular structure in that south-eastern enclosure have been interpreted as a possible gate tower. Together the three enclosures extend north, east, and west of the existing farmhouse and barn buildings on the site.

Carrickmines is perhaps better known to many Dubliners for the controversy surrounding road development in the early 2000s than for the archaeology itself, which makes the surviving and recorded remains all the more worth seeking out carefully. The site sits in the Carrickmines area of south County Dublin, within the administrative remit of Dún Laoghaire-Rathdown County Council. The conservation plan, held at County Hall on Marine Road in Dún Laoghaire, remains the primary document for understanding the enclosures in relation to one another. The south-west enclosure is not the most visually dramatic element of the complex, but in terms of understanding how the Lissebon family gradually elaborated their defences across generations, it is the connective piece that holds the sequence together.