Somewhere in the area of Cardiffsbridge, on the northside of County Dublin, there was once a holy well dedicated to St. Carthage, a site that drew enough local significance to be recorded in writing, yet has since slipped so thoroughly from collective memory that its precise location can no longer be determined.

Holy wells are a feature of the Irish landscape that predate Christianity, though they were readily absorbed into it; most are associated with a named saint and were traditionally visited on that saint's feast day, with patterns of prayer, circumambulation, and offerings of rags or medals tied to nearby branches. The earliest known record of this particular well appears in an 18th century source, cited by the folklorist and scholar Caoimhín Ó Danachair in a 1958 publication, where it is named as St. Carthage's well. Carthage, also known as Carthach or Mo Chuda, was an early Irish monastic figure associated primarily with Lismore in County Waterford and with Rahan in County Offaly, which makes his appearance in a Dublin place-name worth a moment's pause, though dedications to popular saints could travel far from their geographical origins. Beyond that single reference, the documentary record goes quiet.

For anyone curious enough to go looking, the Cardiffsbridge area lies in the Finglas district of north Dublin, now largely absorbed into suburban development. The well was compiled as a monument record by archaeologist Geraldine Stout and uploaded to the national database in August 2011, a small bureaucratic act that preserved at least the memory of the memory. There is no map reference, no field boundary to check, no surviving local tradition noted to guide a search. What remains is the name, the saint, and the knowledge that something was once there, which is, in its own way, a particular kind of historical document.