Built on a low spur running north to south, this fortification once belonged to the Esmond family of nearby Johnstown Castle, who held considerable lands here in the early 17th century. The Civil Survey of 1654-5 records that Marcus Esmond owned 121 acres at Rathlannon in 1641, though the family’s fortunes would change dramatically during the Cromwellian period; John Esmond of Rathlannon was listed for transportation with four dependents in 1653.

The castle itself is a compact but formidable structure, measuring 9 metres northwest to southeast and 7.65 metres northeast to southwest. Its walls rise complete to the stepped crenellations, featuring well-crafted quoins and a slight base batter that helped deflect projectiles. The main entrance, set towards the northern end of the northeast wall, showcases the defensive mindset of its builders; a pointed arch doorway of undressed stone leads into a lobby protected by both machicolation at second floor level and a murder hole above, allowing defenders to rain down unpleasantries on unwelcome visitors. Originally, a mural staircase in the northwest wall provided access between floors, whilst a separate newel stair connected the second storey to the battlements.

Despite its impressive exterior preservation, the interior tells a different story of adaptation and change. The original medieval arrangements have been completely remodelled in brick, likely during the 18th or 19th centuries when the upper three floors were converted to rest on joists set into the northeast and southwest walls. This transformation from military stronghold to more peaceful dwelling reflects the changing fortunes of both the castle and the surrounding landscape. The structure appears on the Down Survey map of 1655-8 and remained part of Rathlannon townland until sometime after 1839, when it began its journey into the historical record as one of Wexford’s enduring medieval monuments.