By 1684, a visitor named Robert Leigh described it as an ancient corporation governed by a master and burghers, yet what he actually found was a ruined castle, a working parish church, the shells of two chapels, and about a dozen cabins. That gap between institutional form and physical reality is a reasonable summary of the settlement's whole trajectory: a town that accumulated significance across centuries without ever quite consolidating it.

The origins lie with a monastery founded by St Munna in the sixth century, the saint's name surviving in the Irish place name Teach Munna, meaning Munna's house. The monastery had apparently collapsed as a functioning institution by the end of the eleventh century, though a convent of nuns had been established at the site before 1170. After the Norman invasion, Strongbow reserved Taghmon as a seigneurial manor, a lord's personal landholding kept outside the general division of conquered territory, attaching to it the parishes of Taghmon and Coolstuff and the extensive surrounding forest. The manor passed through several hands; by 1247 it had gone to Agatha Mortimer, and later inquisitions record the steady business of medieval landholding: burgesses, carucates, and burgage plots, the small units of land that medieval towns typically granted to resident tradespeople and settlers. An inquisition of 1358 records one Roesia, widow of Robert Meyler of Duncormick, holding 21 burgages in the western part of the town. In 1420 the burgage rent of £8 was owed to Sir Gilbert Talbot. The settlement never received a formal charter and was probably never fortified in any serious way. In 1548 William Hore of Harperstown was granted the castle, and in 1600 the town was burned by disaffected Kavanaghs.

What finished Taghmon as a place of any commercial consequence was not violence but roads. Its position on the old route between Wexford town and New Ross had given it whatever modest prosperity it enjoyed, but the construction of the more direct New Line, the road now known as the N25, in the 1830s drew traffic away from it. A later nineteenth-century road running from Wexford town towards Wellington Bridge and Duncannon bypassed the village by roughly four kilometres to the south, compounding the isolation. What remains today is a layered archaeological puzzle: the site of St Munna's church, a high cross, the possible site of St Bridget's church associated with the earlier nunnery, Our Lady's church, a tower house, and two holy wells. More recently, monitoring of a gas pipeline along the east of Main Street revealed a large ditch, four metres wide and a metre deep, almost certainly the boundary enclosure of the early monastery, its fill of dark clayey silt yielding no datable material but outlining, faintly, the shape of something that preceded everything else here.