Beneath the floor of Maynooth Castle's keep, before it was fitted out as an exhibition space, excavators found something that had no business being there: the partial remains of a rectangular prehistoric building, the oldest structure on a site that would later become one of the most significant Anglo-Norman strongholds in Leinster. No artefacts were found directly associated with it, though a stone axe head, an unfinished macehead, and a scattering of flint waste flakes turned up in layers above it, possibly disturbed remnants of whatever life was lived in or around that early structure.

The 1996 excavation, carried out for Dúchas, The Heritage Service, revealed seven distinct phases of occupation compressed into the ground beneath the keep. Over the prehistoric remains, at least two small post-and-wattle round houses, each roughly five metres across, were built in the early medieval period. Post-and-wattle construction is exactly what it sounds like: upright wooden posts woven with flexible branches or rods, plastered with daub to form walls. The hearths and post-holes of these houses yielded good carbon samples for dating, even if no datable finds came with them. The later of the two round houses seems to have acquired a curving wooden stockade on one side, and it appears to have been in use around the time the surrounding land was first being cultivated in organised, regularly spaced furrows. That cultivation eventually overtook the house entirely and continued, apparently unbroken, until the Anglo-Normans arrived. The site seems to have passed into Norman hands around 1175, at which point the archaeology shifts register: a low sod mound about a metre high was raised, topped by another rectangular post-and-wattle building and enclosed by a stout wooden fence with an east-facing entrance. The finds from this level include an arrowhead, an iron spur, a scabbard chape, and pottery identified as Ham Green ware, a type of glazed ceramic produced near Bristol and commonly associated with early Anglo-Norman settlement in Ireland. The stone keep followed, probably in the late 1180s, its construction visible in the archaeological record as thick spreads of mortar. The original structure was divided into two rooms by three piers carrying the first floor.

What makes the site quietly remarkable is the layering: a prehistoric building giving way to early medieval round houses, giving way to cultivation, giving way to a Norman mound, giving way to one of the country's better-known medieval keeps, all of it stacked within the footprint of a single ground floor. The castle itself is accessible in Maynooth town, and the keep interior, now an exhibition space, sits directly above the ground where all of this was found.