The name of the townland, and by extension the church, points firmly toward St Ambrose, the fourth-century Roman aristocrat who became bishop of Milan in AD 374. It is a plausible enough reading, but the antiquarian John O'Donovan, writing around 1840, considered it most unlikely. A medieval document lends weight to his scepticism: a record from 1408 describes Thomas Founreys, rector of Rathmore in County Meath, as also serving as chaplain of 'the Blessed Mary of [Am]brosetoun, diocese of Ferns'. That phrasing suggests the church was dedicated not to Ambrose at all, but to the Virgin Mary, and that the saint's name attached itself to the place by association with the settlement rather than with any devotional intention.

By 1615, when Thomas Ram, the Protestant bishop of Ferns, conducted a formal visitation of his diocese, the church was still functional. Ram's record notes that William Underwood, described as a student, held the rectorship, while Henry Reigh served as curate, and that both the nave and chancel were in a reasonable state of repair. The building they knew was a simple undivided nave and chancel structure, measuring roughly 17.6 metres along its east-west axis and just under 7 metres wide. Today almost nothing of it stands. The east gable survives to a height of about 4.6 metres and contains a narrow granite lancet window, a tall pointed opening of the kind common in medieval Irish parish churches, just 35 centimetres wide and 1.37 metres tall, which once held glazing bars. The foundations of the north and south walls lie beneath the grass, but the west end has vanished entirely, partly obscured by quarrying that has also eaten into the circular earthen bank and masonry wall enclosing the graveyard. A rectangular granite font was at one point reported inside the ruin, though this turns out to be a case of mistaken identity; it belongs to a different church altogether. Some 30 metres to the north-west, O'Donovan recorded the site of St Ambrose's Well, where patterns, the traditional Irish gatherings of prayer and festivity at holy wells, were once held on a date that is no longer known. The quarrying that damaged the churchyard boundary has also obliterated the well, and no trace of it remains.