The site goes by an older name too, Friarstown, a toponym that quietly preserves the memory of what once stood there even as every stone of the original buildings has long since vanished.

The Knights Hospitaller, formally the Order of the Hospital of Saint John of Jerusalem, were a military religious order that combined the care of pilgrims and the sick with armed service in the Crusading era. Their Irish properties were administered from their priory at Kilmainham in Dublin. The Killerrig house was founded by Gilbert de Borand and confirmed to the order in 1212, according to the Registrum de Kilmainham, the administrative register of that mother house. By 1540, when Henrician commissioners were surveying dissolved religious properties, jurors recorded at the place then called Frerton a ruined castle and three messuages, the latter being ordinary dwellings or domestic properties attached to the site. The fact that the castle was already described as ruined at that point makes the next piece of evidence more surprising: the Down Survey, the great mid-seventeenth-century mapping project ordered by the Cromwellian administration, records a castle here in good repair as late as 1650. Whether a structure had been rebuilt in the intervening century, or whether the two records refer to different assessments of the same fabric, is not clear from what survives.

Today there is no visible trace of any medieval structure at the site. What remains is a working farmyard, and the history of the place is preserved only in documents rather than in any fabric above ground.