The ruins of St Mary's Church are among the more substantial remnants of that era still visible in the town, a roofless shell of cut stone that speaks to a community once wealthy enough, and numerous enough, to build ambitiously in an age when such building was a serious undertaking.

The town itself was founded in the thirteenth century by Thomas FitzAnthony Walsh, a seneschal of Leinster, from whom it takes its name. At its height it was a walled settlement with several religious foundations, and the parish church reflects that period of relative prosperity. Medieval parish churches of this type were typically simple in plan, a nave and chancel arrangement, sometimes extended over generations as congregations grew or patrons wished to leave their mark. Thomastown's position on the Nore made it a commercial hub, and the wealth that flowed through it left traces in stone that have outlasted the merchants and officials who generated it.