The early nineteenth-century building of St Mary's Church of Ireland sits south of the River Robe on a site where a medieval church dedicated to St Mary and Holy Cross once stood, yet not a single visible stone of that earlier structure remains above ground. The medieval fabric has been absorbed or erased, except for a few suggestive details that surfaced only when the building was stripped back during renovations in 1993 and 1994.

The first written mention of the church comes from a papal letter of 1413, in which a man named Henry is identified as vicar. He had already been driven out by the following year, the church described as without a cure and without rents to support a rector, a condition so dire that the Pope offered a relaxation of penance to anyone who visited and gave alms on appointed days. Ballinrobe itself had been an Anglo-Norman borough since sometime after 1237, and by the early 1400s it had become the principal stronghold of the Mac William Íochtar, the leading branch of the Burke clan in Connacht. In 1655, following the Cromwellian conquest, the town was granted to one James Cuff, and the church and its graveyard passed into Protestant use. The current building was put up in the early 1800s with funding from the Board of First Fruits, a body that financed Church of Ireland construction across Ireland during that period. During the 1993 renovations, two graveslabs dated 1718 and 1719 were lifted from the church floor, and a wall monument from 1668, presumably displaced during the earlier rebuilding, was found set into the exterior north wall. These pieces suggest the graveyard and its predecessor church remained in continuous use across the centuries of transition.

The most intriguing fragment to emerge during those same renovations was a piscina, a small carved basin set into a wall that priests used to wash their hands and rinse sacred vessels during Mass, cut from a square limestone block measuring roughly 45 centimetres across. What made it unusual was its drainage: rather than the single hole that is standard in such fittings, this one had five. A photograph taken during the work also revealed the ghost of a gothic arch infilled with later masonry in the north wall, a detail no longer visible now that the renovations are complete. The piscina itself was removed from the building and its current whereabouts are unknown.