Built atop a commanding northwest-southeast ridge, this National Monument began life as a motte and bailey castle constructed in 1193, just a few years after the Anglo-Normans peacefully settled much of what became County Louth following the death of Murrough O'Carroll, king of Oriel. The original earthwork fortress was part of Peter Pipard's ambitious grant around 1190, which included not only the remote baronies of Farney but also Cremorne and Dartree, all administered from Ardee.

The castle's evolution from timber to stone reflects the ongoing struggle to maintain control over this borderland. After the original castle burned down, Ralph FitzNicholas, custodian of the Pipard minors, ordered the construction of a stone castle between 1228 and 1234, completing it by 1244. Today, visitors can still see remnants of this ambitious fortification: a truncated conical motte roughly 45 metres in diameter at its base and rising between 8.8 and 12 metres high, topped with the ruins of masonry walls about 1.5 metres thick. The rectangular summit, measuring approximately 17.8 by 15 metres, preserves fragments of the castle's internal structure, including a portion of a cross-wall and a fallen barrel vault fragment showing plank centring on its underside. A stone-faced causeway, complete with traces of its original wall, spans the fosse to connect the motte with an inner bailey where excavations have uncovered evidence of what may have been a wooden bridge.

Despite these fortifications, holding Farney proved impossible for the Anglo-Norman lords. The Pipards surrendered the territory back to the Crown in 1302, effectively abandoning it to Irish control. Subsequent attempts to re-establish authority through various leases; to Walter de Cusack in 1317, John Clinton in 1333, and Roger de Gernon in 1372, who was specifically instructed to rebuild the castle; all failed. By 1401, even Eochaidh Mac Mahon held a lease, though pointedly excluding Donaghmoyne castle itself. The last mention of the castle in any lease dates to 1414, and by the time the Earl of Essex took control of Farney in 1572, the fortress had been so thoroughly forgotten that when his successor built a new stronghold for the barony between 1628 and 1633, he chose Carrickmacross instead, leaving Mannan Castle to stand as a monument to medieval frontier warfare.