A small stone-walled, thatched structure was built against the inner face of the south-east wall of the local tower house, sitting at roughly first-floor level, and whoever constructed it converted an existing window embrasure into an altar shelf by filling in its lower section. The shelf measures just under a metre and three-quarters wide, and the whole interior of the building runs to less than six metres by four. It is a cramped, quietly ingenious arrangement, and it sits on a bluff overlooking a shallow valley, with two holy wells, St James' Well and St Anne's Well, lying within forty metres of the building.

Mass-houses were the improvised Catholic places of worship that appeared across Ireland during the Penal era, when public Catholic worship was suppressed under a series of laws introduced in the late seventeenth century. Priests said Mass in barns, beside outdoor rocks, or in whatever shelter could be found or quietly constructed. The Tomhaggard example fits that pattern, though its precise origins are not entirely clear. A Jasper Devereux was registered as pastor of Mayglass and Tomhaggard in 1703, but the parish does not appear in a later survey of Catholic clergy in the Ferns diocese from 1731, which leaves a gap in what can be known about who used the building and for how long. By 1839, when the first edition of the Ordnance Survey six-inch map was produced, the structure was marked but left unnamed. Over the doorway, a D-shaped stone carries an unusual carved decoration, four curves along its rounded edge and a T-shaped incision, the meaning of which is not recorded. A small excavation carried out in 2003 in connection with underpinning work on a buttress on the north side of the building produced no material that shed further light on the history of the site.