The present-day layout of Ballymore Eustace, a quiet village on the Kildare side of the River Liffey near the Wicklow border, gives little indication that it was once a functioning medieval borough complete with weekly markets, annual fairs, water mills, and 160 recorded burgesses. A 1326 survey of the manor lists all of this with administrative precision, including a common kiln, sixteen rent-free cottages, four cottages described as lying in waste, and a plot of land bearing the evocative name 'Taillourbourgage'. That level of documented urban activity makes the uncertainty about where exactly the medieval town stood all the more striking. Its core may have been near the old church at the east end of the present settlement, or it may have been organised along Church Road to the southwest, where long, narrow plots running off the road still follow a pattern consistent with burgage tenure, the system by which medieval town-dwellers held land in exchange for a fixed annual rent.

The town's origins lie in the archiepiscopal estates of Dublin. A grant of 1181 to 1190 refers to the archbishop's castle here, and a further grant of 1191 mentions his manor of Ballymore, the name derived from the Irish Baile mór, meaning simply 'big town', which appears in late twelfth-century documents. The Eustace family, who held the constableship of the castle for much of the medieval period, gave the settlement the second half of its name, though that epithet does not appear in English sources until 1666. A Norman motte, the earthen mound on which a timber castle would have been raised, was erected probably to the north of the present town. Between 1189 and 1199 the archbishop was granted a weekly market; by 1234 Henry III had added the right to a yearly fair; and by the mid-thirteenth century the place had the formal status of a borough, with burgesses paying twelve pence annually and holding twelve acres each. The sixteenth century brought repeated violence: the town was plundered during the Geraldine revolt of 1546, and in 1572 it was reported that almost everything had been burned, with only one le Strange's house and castle left standing. By 1659 the recorded population had fallen to 132. James I had by then already revived the market and fair rights in 1608, and the Main Street and Market Square area of the present village is thought to reflect a seventeenth-century rebuilding rather than any medieval plan.

At the east end of the town, the graveyard of St. John's Church contains two high crosses and a cross slab, all suggesting a pre-Norman ecclesiastical presence of some significance. High crosses of this type were typically carved in the early medieval period and often marked important monastic or church sites. Archaeological testing carried out in 1997 along Bishopslane Road, in the ground between the medieval church and the motte castle site, turned up medieval features, pottery dating from the twelfth to the fifteenth century, and traces of possible structures separated by cultivation furrows, a physical reminder that the borough's streets and plots, however well-documented on parchment, are only beginning to be located in the soil beneath the present village.