Their story begins around 1204 when Adam Sinad surrendered land in Kilkenny, likely in exchange for the Ballybrennan estate, which William Synach held as a knight’s fee from the Valency estate by 1247. The family consolidated their position through various land grants, including one from Gerald de Rupe around 1230, which gave them a foothold in Fernegenel and possibly led to their later claims on nearby Ballytramon in the 16th century.

By the time of Richard Synnott’s death in 1591, the family had established a castle at Ballybrennan, though no contemporary records specifically mention it. The property passed through Walter to Martin, and a castle structure appears clearly marked on the Down Survey barony map created between 1656 and 1658. The turbulent Cromwellian period brought significant challenges; Richard Synnott of Ballybrennan was listed for transplantation to Connaught in 1653 along with 21 dependents, whilst records show that Jasper Synnott had owned 560 acres across multiple townlands in Ballybrennan parish before the upheavals of 1641.

Today, the castle’s remains tell a quieter story. Located on level, low-lying ground near a small stream and not far from the original shoreline of South Wexford harbour, the structure was reportedly dismantled by a tenant named R. Jones sometime before 1837, when it belonged to the Earl of Rathdown. What survives is incorporated into the area just east of the present Ballybrennan House; walls approximately one metre thick rise two storeys, crowned with decorative false crenelations on the south front. A narrow pointed window in the south wall, though set in brick, may be one of the few original medieval features still visible. Nearby, a windmill stands about 90 metres to the south-southeast, whilst Ballybrennan church lies roughly 200 metres southwest, creating a small cluster of historical sites that mark centuries of continuous occupation.