Pressed against the north side of the nave, this three-storey limestone structure was not a bell tower or a defensive keep in any military sense; it was a residence, complete with fireplaces, a garderobe (a small medieval latrine built into the wall), vaulted floors, and a stair turret rising from the north-west corner. The ground floor retains traces of wicker-work centring, the imprint left in the stone when curved timber and woven rods were used as a mould to cast the barrel vault above. Someone, at some point, was living in considerable proximity to the dead.

The church itself was in existence by around 1219, when it was granted to the Augustinian priory of St. Catherine, founded by one Warisius de Pech to the west of Lucan, a place whose Irish name, Leamhcán, means marshy place. By 1268, a charter confirmed the grant of St. Mary's to the canons regular of St. Catherine, witnessed by an impressive roll call that included the Archbishop of Dublin, the Bishops of Ossory and Kildare, and Hugh de Braunton, constable of Lucan. The 1302 to 1307 Ecclesiastical Taxation of the Deanery of Taney valued the church at £20 per annum. Ownership shifted repeatedly across the medieval centuries: when St. Catherine's was united with the Augustinian Abbey of St. Thomas in Dublin in 1337, the advowson, meaning the right to appoint the parish priest, passed with it. After the Dissolution of the Monasteries in 1540 to 1541, the rectory's tithes passed through the hands of the Earl of Kildare, were forfeited following his attainder for high treason, and were then granted to Sir Thomas Luttrell and Patrick Barnewall. By 1615, a Royal Visitation found the church and chancel in good order, with books. By 1630, Archbishop Bulkeley's visitors reported the chancel ruinous and noted, with a certain weariness, that there were not above five people in the parish attending services. The vicar's wife, they added, was a recusant.

The ruins sit on the north-east side of a semi-circular walled graveyard on the demesne lands of Lucan House, which stands roughly 80 metres to the west. The church is a simple nave and chancel structure of uncoursed rubble limestone, missing its north nave wall but retaining a near-complete west gable with the remains of a bellcote and a fine cusped ogee-headed double-light window, the ogee being the S-curved arch form characteristic of late medieval Irish ecclesiastical stonework. The original south doorway is blocked. The residential tower to the north rewards close attention: look for the decorated head of an ogee window reused above a later inserted entrance, and for the first-floor garderobe tucked beside a lintelled opening. The site is within the demesne grounds, and access should be confirmed before visiting.