The site’s strategic position first attracted Hugh de Lacy’s attention when he granted Dunboyne to William le Petit, who established a ringwork castle on the south bank of the stream, possibly repurposing an existing rath. This early fortification, whilst practical, proved insufficient for the evolving needs of medieval lordship and may have been rebuilt in stone at some point.

The transformation from earthwork to stone castle gained royal impetus in 1475–6 when Edward IV decreed that a new castle should be erected at Dunboyne within a year, most likely taking the form of a tower house. This medieval structure appears in historical records, with the Civil Survey of 1654 noting one stone house owned by Lord Dunboyne, whilst the Down Survey of 1656–8 depicts a two-storey gabled building, apparently roofless by then, standing on the stream’s southern bank. Archaeological investigations have uncovered tantalising glimpses of this medieval past; excavations on the north side of the current house revealed a substantial wall, 1.8 metres wide and up to 1.6 metres high, with an associated stone floor, likely remnants of the castle mentioned in the Statute Rolls. Beneath later cobbling, archaeologists also discovered seventeenth-century gravel-tempered pottery, suggesting continuous occupation through turbulent times.

The castle visitors see today represents yet another chapter in this long story, built by the tenth or eleventh Baron of Dunboyne during the 1770s or 1780s, with further modifications in the 1830s. This Georgian mansion replaced its medieval predecessor but maintained the site’s centuries-old connection to the Dunboyne lordship. Additional archaeological testing has revealed another medieval building outside the main enclosure to the east, suggesting the original castle complex was more extensive than previously thought, with multiple structures serving the needs of a medieval lordship that controlled this strategic crossing of the Castle River.