The most visible trace is a disused Church of Ireland building dating from 1817, but built into its western end is a tower that belongs to a far earlier structure, the parish church of the royal manor of Crumlin. To the east of that church, a field once held what may have been a motte castle, the kind of earthen mound topped with a timber or stone fortification that the Normans used to anchor their grip on newly acquired territory. Rocque's map of 1760 still shows a cluster of dwellings gathered around the church, and the seventeenth-century Down Survey map of Newcastle and Uppercross barony depicts the church alongside two other buildings that may represent the castle and manor house, all sitting beside a medieval routeway marked with a dotted line.

The manor's origins run deep. Before the death of Archbishop Laurence O'Toole in 1180, a man named Osbert Trussell witnessed deeds relating to a land grant in north County Dublin; his descendants became prominent tenants on the manor and eventually took Crumlin as their surname. By 1212, one William Crumlin was paying the substantial sum of £16 in pepper for the lands, possibly clearing years of arrears. In 1216, the manor was held by William Fitz John of Harptree, whose family appears to have been descended from the owners of Richmont Castle in Harptree Coombe in Somerset. In 1336, the king granted the manor to John Fitzwilliam Jordan for life as an ancient royal demesne, and in the following year offered the local tenants 122 acres at a rent of £6 17s. 3d., on condition that they enclosed the town against attack within ten years, at their own expense. The manor was also a prebend of St Patrick's Cathedral, meaning the income from its parish church supported a cathedral clergyman. By 1528, Gerald Fitzgerald, ninth earl of Kildare, held a castle in Crumlin along with one messuage and a hundred acres, and land here was still being called 'Earl's ground' as late as 1593. The Court of Crumlin's surviving records add quieter textures: a 1495 deed records two messuages and fifteen and a half acres passing between a yeoman and a husbandman, with the property ultimately bequeathed in frankalmoign, a form of tenure by which land was held by a church in return for spiritual services, to keep a wax light burning before the image of the Virgin Mary forever. A 1594 court order sets out, in practical detail, how many labourers each class of tenant must provide for cleaning the town ditch.

The medieval settlement cluster is listed in Dublin's Record of Monuments and Places, and the zone of probable archaeological significance is centred on the area around the old church on Crumlin Road. The disused St Mary's is not generally open, but its exterior, including the medieval tower incorporated into the west end, can be observed from the road. The precise location of the sixteenth-century castle recorded in the documentary sources has never been established, so the surrounding area retains a degree of genuine archaeological uncertainty. Those interested in the cartographic evidence can consult the Down Survey maps and Rocque's 1760 survey online through various digitisation projects, where the earlier settlement pattern becomes legible against what the area looks like today.