Town walls of this kind were a serious undertaking in medieval Ireland, typically comprising a curtain wall with mural towers, gatehouses, and sometimes a defensive ditch or fosse on the outer side. They marked the limits of civic authority, defined who was inside and who was out, and required ongoing maintenance and communal investment to remain effective.

Thomastown itself was founded in the thirteenth century by Thomas FitzAnthony, a Welsh settler and seneschal of Leinster, from whom the town takes its name. It grew into a place of some regional importance during the medieval period, acquiring a merchant class, a Franciscan friary, and the kind of civic infrastructure that a town wall both symbolised and protected. The defences would have enclosed this activity, separating the burgess town from the surrounding countryside. Over subsequent centuries, as the strategic and commercial importance of the town shifted, the walls fell out of use and were gradually absorbed into the fabric of later construction, which is why they tend to be noticed only in passing rather than as a formal monument.

Visitors with an eye for it can pick out sections of the old masonry in various parts of the town, particularly along the western and northern margins. The stonework tends to be older and thicker than what surrounds it, and occasionally a stretch will rear up to a surprising height before dissolving back into a more modern boundary. The Franciscan friary ruins nearby offer a complementary sense of the medieval town's spatial logic, and the two are worth taking in together on foot.