A document from 1316, preserved in the Ormond Deeds, records a burgage plot, one of the long, narrow land parcels typical of a Norman planned town, stretching from the regia strata, the king's road, all the way back to "the water of Avenlyffe", meaning the Liffey. Historians once debated whether the modern street occupied the same ground as its predecessor, but that single deed settled the argument: the road has not moved. What has shifted almost beyond recognition is the town around it, and yet the basic logic of the place, one principal street running east to west, a church at the western fringe, the river at the back of the plots, remains legible if you know what to look for.

Lucan is unusual among Co. Dublin's medieval boroughs in that it owes nothing to pre-Norman settlement and was not established by the archbishops of Dublin, who founded several other towns in the county. It began with a royal grant: around 1185 the Ormond Deeds identify Alard FitzWilliam as the first holder of the manor of Leuekan, by gift of King John, then Lord of Ireland. By 1203 to 1204, the manor had passed, for the price of forty marks and a palfrey, a riding horse offered as a kind of fine, to Warrisius de Peche, whose family held it for much of the thirteenth century. The FitzGerald earls of Kildare controlled it during the later medieval period, and after their fall following the Silken Thomas rebellion of the 1530s, Sir William Sarsfield purchased it. The Sarsfields were dispossessed under Cromwell, restored under James II, and it was in that brief window that Patrick Sarsfield was created earl of Lucan. The town found an unlikely second act in 1758, when a spa was discovered there, drawing visitors throughout the late eighteenth century before that trade too faded away. The castle that would have anchored the medieval manor is a persistent puzzle: the residential tower attached to the parish church was mistaken for it by almost every previous writer, and its probable location near the present Lucan House remains unconfirmed.

The medieval street plan is best appreciated with a period map in hand. Taylor's 1816 map of the environs of Dublin shows the village, the bridge over the Liffey, and the old mill, though the manorial mill associated with the manor was more likely located at Lutterelstown than at Lucan itself. The Down Survey map of Newcastle Barony, held by Trinity College Dublin, shows Lucan House sitting beside the medieval road, flanked by dwellings and the parish church, a snapshot of the settlement before later development obscured the pattern. Bridge Street, running down to the Liffey crossing first mentioned in the seventeenth century, lacks the burgage plots that line Main Street on either side, a small but telling sign that it was added to the town later rather than forming part of the original Norman layout.