Before any castle, there was a hermitage. The old Irish name Disert-Iolladhan - Iolladhan's hermitage, sometimes anglicised as Disertillan - belongs to a priest who walked these riverbanks in the sixth century. Iolladhan, according to the genealogies preserved in the Martyrology of Tallaght, was a great-grandson of King Cormac of Leinster, who abdicated his crown in 515 and died a monk at Bangor. The family produced kings and abbots in equal measure. Iolladhan's feast day falls on 2 February, marking him among the Christian saints of early Ireland. Over centuries the Irish word disert, meaning a hermit's retreat, was traded for the harder Norman word castle, and Iolladhan's quiet name dressed itself in stone. The hermitage itself, like its founder, has long since slipped beneath the green Kildare turf.

In 1271 a Norman lord named William de Mandesham took possession of these lands, and by the early fourteenth century the de Hereford and Rochford families had raised a tower house on the site. It was a working castle, not a romantic ruin, controlling a slice of country between Dublin and the Liffey. The wars of 1641 and 1642 left it burned and broken. A Mrs Bowell patched it back together in the 1650s. By the eighteenth century, with the great Anglo-Irish estates rising all around, the old tower had become a quarry: local builders carried its stones away one cartload at a time, and by the early 2000s archaeologists working a development survey could only recover pottery sherds from a hollow that had been deliberately filled in. In 1557, a Patrick Sarsfield of Tisteldalen - great-great-granduncle of the famous Patrick Sarsfield, Earl of Lucan and Jacobite commander - obtained a pardon here from the English colonial government. The Sarsfields would echo through Irish history; this branch of them simply faded into it.

Among the ruins one object survived intact, and it left Castledillon for the visitor centre in Kildare town. The Castledillon Friar's Stone is an incised slab carved with the figure of a priest. His left hand holds a reliquary on a chain. His right rests palm-down on his chest. The medieval French inscription - ICI GIST DEU DE SA ALME EIT MERCI, "Here Lies, God Have Mercy on His Soul" - leaves out the name itself. Local folklore made him a bishop, but he carries no crosier, so historians now think him an abbot of nearby St Wolstan's, perhaps one of the last men to hold that office before Henry VIII dissolved the friary in 1541. He outlived his religious house and was laid to rest in a country churchyard four miles away, his name eroded but his face still looking out from the stone.

Castledillon's last act of public history came during the First World War. The British Army leased a stretch of McKenna's land called "The Butts" and turned it into a firing range. Recruits drilled in the mud, far from the streets of Dublin or the villages of Leinster they had left behind. A folk song from the period captures the loneliness with a soldier's dark humour: "There is an isolated, desolated spot I'd like to mention / Where all the folks quick march or stand to attention / It's miles away from anywhere, bedad it is a rum one / A chap lived there for fifty years and never saw a woman." A second verse proposed exiling Kaiser Wilhelm there as the cruelest possible fate. Today the firing range is gone. Ann Spellicy's headstone still leans alone among the stones of the old graveyard, the last visible witness to a place that has been called a parish for fourteen hundred years.