The main road through the village bends northward while the southern road curves away in the opposite direction, and the former medieval church sits centrally between the two. Taken together, those curves trace something close to a circular or oval shape, the characteristic form of an early monastic enclosure in Ireland, where the original boundary ditch or bank dictated the paths that later generations of travellers would follow without ever knowing why.

The full ancient name of the place, recorded as Cill Manach Droichit, meaning Kilmanagh of the Bridge, points to a settlement that was already old when it acquired its name. According to the historian William Carrigan, writing in 1905, the monastery was founded around the year 500 by St. Natál, most probably a son of Aenghus mac Nadfraoich, King of Cashel, who was slain in 489. Natál became the local patron saint, his feast day falling on the 31st of July. The Annals of the Four Masters contain several entries from between 780 and 843 that almost certainly refer to this monastery, suggesting it remained active and significant for centuries. By 1307 the settlement had grown into something with genuine urban ambitions; a document from that year refers to the burgesses of Kilmanagh, indicating it had been granted borough status, and it appears on later maps of medieval towns in the county. A field name recorded in the area, Clais an bhaile, meaning the village trench, now filled up, hints at a formal boundary that once defined the settlement's edge. By the mid-seventeenth century, when the Down Survey mapped the barony of Crannagh during the 1650s, the townland of Killmannagh contained just three houses and a church, with the Countess of Ormond listed as proprietor in 1640. The medieval church itself was eventually replaced by a Church of Ireland building in the late eighteenth century.