The broad open space that runs through the modern settlement was originally laid out not as a thoroughfare but as the ground where markets and livestock fairs were held, and the buildings that grew up around it on the south and west sides confirm the arrangement visible on the Ordnance Survey map of 1835. It is an unusually candid piece of urban archaeology: the shape of a medieval marketplace preserved in plain sight, in a place that was never quite a town.

The settlement's ambiguous status goes back a long way. The name Dunboyne contains the Gaelic dún, meaning a fort or strong point, and the second element has been read variously as the personal name Beithin, a reference to the River Boyne, or the word buinne meaning 'yellow river'. Whatever its origins, the place had a recognisable civic life long before it had any formal standing. It was granted the right to hold a weekly market in 1226 and an annual fair from 1229, and by 1423 there is a recorded Provost or mayor alongside the commons, suggesting something close to a corporate identity, even though the settlement was never incorporated and never received a charter. The barony itself had been granted by Hugh de Lacy around 1172 to William le Petit, whose main seat was at Mullingar. The connection to Mullingar ran deep: in 1227 Ralph Petit, bishop of Meath, endowed the Augustinian priory he founded there with the ecclesiastical properties of Dunboyne parish. The Petit interest passed by marriage to the Butlers during the reign of Edward II, when Thomas Butler, brother of the Earl of Ormonde, married Sinolda, the heiress of Sir William Petit. The title of Dunboyne has remained with the Butlers ever since.

The castle that anchored the whole settlement sits across the Castle Stream in Castlefarm townland, on a slight rise about a hundred metres from the water. It began as an earthen ringwork castle, the type constructed by piling up a circular bank and ditch as a defensive enclosure, and was later replaced by a tower house and associated structures in the later medieval period. The Civil Survey of 1654 gives a glimpse of how modest the place remained: only one stone house, belonging to Lord Dunboyne, and a mill are specifically noted, though 42 tenements were recorded. The stream below the bridge appears unusually wide on the Ordnance Survey maps of 1836 and 1908, possibly the trace of a mill-pond that once served that mill. The July horse and cattle fair continued into the nineteenth century, held on the same green that still gives the street its particular width and shape.