But tucked into its south-eastern corner, in the section designated Block Z, the oldest part of the complex, stands a ruined early medieval church that predates the town beside it by well over a thousand years. The contrast is quietly disorienting: rows of contemporary headstones pressing up against masonry that was already ancient when Tralee itself was young.

The site takes its name from the Irish An Ráth Theas, meaning the Southern Fort, and in early literary sources it appears as Ráth Muighe Deiscirt, the Fort of the Southern Plain, a name recorded by O'Donovan in 1841. The ecclesiastical foundation is thought to date to the 7th or 8th century AD, a dating supported by the form of the church itself and by two objects now housed within the building: an ogham stone, one of the inscribed standing stones used in early medieval Ireland to record names in a notched alphabet derived from Latin, and a cross-slab. The church was later modified during the Romanesque period, when a chancel was added, extending the building in a style common across Ireland from roughly the 12th century. A 2012 survey recorded 5,097 headstones across the graveyard as a whole, making it the largest in County Kerry, along with 105 above-ground tombs of chest, box, and strong-box varieties, seven horizontal graveslabs known as ledgers, and several wrought and cast iron crosses. An Air Corps aerial photograph from May 1949 shows the graveyard at that point still largely surrounded by open fields, with several new burial blocks already laid out but as yet unoccupied, a snapshot of the expansion that has continued steadily ever since. The church and its immediate precinct are a National Monument in state ownership.