Running alongside the grounds of Clongoweswood College in County Kildare, a low, overgrown earthen bank traces what was once one of the most politically charged boundaries in medieval Ireland. It is not much to look at now, a broad ridge of turf with ditches on either side, but this is a surviving fragment of the Pale, the earthwork frontier that divided the world of the Anglo-Norman colonial settlement from the Gaelic Irish beyond. Two sections survive here: a northern portion traceable for around 550 metres, between three and four metres wide and rising to about 0.75 metres, with a footpath worn along its crest; and a more substantial southern portion running for roughly 900 metres, averaging five metres in width and reaching three metres in external height. The southern section appears to end at the small, north-westward-flowing Gollymochy River, though a slighter raised bank continues from that point, curving south-east along the riverbank for a further 200 metres or so, and may represent a continuation of the same earthwork.

The Pale, as a concept, took shape gradually. The word itself derives from the Latin palus, meaning a stake, and was also the name given to an English-held fortified zone around Calais in France. In Ireland, it began not as a physical barrier but as a political geography: from the early fourteenth century, Norman settlers increasingly concentrated in counties Dublin, Kildare, Louth, and Meath, pulling back from territory they could no longer hold against Gaelic resurgence. By 1435, chronic raiding, chiefly for cattle, prompted moves towards a more formalised linear boundary. A 1488 Act of Parliament defined its intended course in detail, running from Merrion through Saggard, Rathcoole, Kilteel, Rathmore, and Ballymore Eustace, and on through Harristown, Naas, Clane, and Kilcock. In 1494, Parliament ordered every landowner and farmer along the marches, the borderlands, to construct a double ditch six feet high on whichever side of their land faced the Irish. It is a scholar named Ellis who points to a statute of Poynings' parliament in 1495 as the first time the term 'pale' was formally applied to Ireland itself. The boundary contracted eastwards over the following decades, and it is unclear whether the full line was ever completely ditched. By the seventeenth century it had ceased to function as either a defensive or political boundary in any meaningful sense.

Very few sections have been positively identified in County Kildare, which makes the Castlebrown portions particularly valuable. When a sewage pipeline was laid through a modern breach in the southern section in 1993, a watching brief recorded a cross-section of the earthwork in detail. The full structure at that point measured 8.5 metres across. The western fosse, a fosse being the ditch on the outer, more exposed face of a defensive bank, was V-shaped, two metres wide and over a metre deep. The flat-topped central bank rose 2.2 metres above the bottom of that ditch. The eastern fosse was shallower and wider, with a rounded profile. No artefacts were found, which is unsurprising for a feature that was essentially an agricultural and civil engineering project rather than a site of occupation.