But beneath that later structure, and largely invisible to anyone passing along the road beside the Liffey, lies the ghost of an earlier house altogether, one that appears on the Down Survey map of Newcastle Barony in the mid-17th century as a substantial gable-fronted building with tall lateral chimneystacks and a central tower, drawn in the manner typical of late 16th- and early 17th-century domestic architecture in Ireland. The Down Survey, a remarkable land-mapping project carried out in the 1650s to assist Cromwellian land redistribution, places this house to the south of Lucan Bridge and alongside the medieval road through the village. The 1654 to 1656 Civil Survey is equally direct, recording at Lucan 'a verry faire house, a Chappell, some thatch houses and Cabbins, and a good Stone bridge crossing the river Liffey.'

The manor of Lucan had been granted to Sir William Sarsfield from the Earl of Kildare in 1569, when the Council of Ireland certified the transaction to Queen Elizabeth. By 1641 the house was the residence of William Sarsfield, recorded as an Irish Catholic landowner with 947 acres in Lucan. The Cromwellian settlement disrupted all of this: in 1653, Colonel Theophilus Jones petitioned the Council of State for a lease over the Lucan lands, claiming them in lieu of arrears owed to him for parliamentary service. What followed was decades of legal contest. William Sarsfield died in 1675, leaving a young daughter, Charlotte, whose mother was Mary, sister to the Duke of Monmouth. Charles II had granted the Lucan estate to William partly as a marriage portion connected to that union. After the Williamite wars, Colonel Patrick Sarsfield, younger brother of Charlotte's father, used the upheaval to seize possession of the estate, exploiting Charlotte's minority and her absence in England. A rejected claim lodged in 1702 by a John Green at Chichester House, College Green, described the property as 'the Castle, and great white house at Lucan,' a phrase that suggests the 17th-century house stood beside or was attached to the even older medieval castle of the Lucan manor. Charlotte's title was eventually confirmed by Letters of Privy-Seal in April 1696, and by 1698 she and her husband Agmondisham Vesey were petitioning Parliament to ensure their restored title was not undone by broader forfeiture legislation.

The site today is within the grounds of Lucan House, which functions as the Italian Ambassador's official residence and is not generally open to the public. The Down Survey map, however, is freely accessible through the Down Survey Project online, and tracing the outline of the earlier house against the present demesne is a worthwhile exercise in itself. The medieval bridge that once flanked the house survives in modified form, and the general configuration of the village as shown on the 17th-century map, road, bridge, and buildings clustered along the Liffey, remains legible in the modern streetscape if you know what you are looking for.