The most striking object inside the ruin is a carved stone cross-slab, 1.42 metres tall, that may predate the medieval parish church by several centuries and is now the sole visible remnant of what was almost certainly an Early Christian foundation. A cross-slab is a slab of stone carved with a cross on one or both faces, distinct from a free-standing high cross, and this example is carved on both sides with Latin crosses whose arms end in expanded terminals. On the southern face, the base of the cross terminates in a spiral elaboration, and a small crescent or loop beside the head of the cross represents the chi-rho symbol, the ancient monogram of Christ formed from the first two letters of his name in Greek. Part of the stone's top and western edge have broken away, and the head and right arm of the cross on the northern face are lost.

The site takes its name from the Irish Cill Seanaigh, meaning the church of Seanach, and is associated with St. Senach, the same figure commemorated at the early church on Illauntannig, one of the Magharees islands lying just offshore. A parish church existed here by 1302, but by 1473 it was recorded as having 'long been void', meaning it had already fallen out of use for some considerable time. The cross-slab was not always where it now stands. By the time Frances Henry documented it in 1937, it occupied the north-west corner of the church; it has since been moved to a position directly west of the doorway, and its original location within or around the site is unknown. About 350 metres north along the shoreline, a large boulder known as Cloch an Turais was once a destination for pilgrims on Good Fridays, a tradition recorded by An Seabhac in 1939. The boulder and the ruined church together suggest a layering of devotional practice across this small coastal strip, one reaching back through the medieval parish into the earlier Christian centuries and perhaps beyond.