This 16th-century sandstone monument measures nearly two metres long and tapers from 72cm to 46cm wide, its trapezoidal form typical of coffin-shaped gravestones from the period. The slab made quite a journey to reach its current home; in 1868, it was transported from the ruins of the Franciscan friary at Ballysaggart, where it had likely marked the resting place of someone significant within the McSwyne gallowglass dynasty, possibly Niall Mor MacSweeney himself.

The slab's surface tells a complex story through nine carved panels separated by bands of ribbon interlacing, all executed in shallow false relief that has weathered over the centuries. The most striking feature appears in the uppermost left panel: a helmeted and plumed gallowglass warrior, complete with battle axe in his right hand and sword by his side. He wears what appears to be a jack or coat of mail extending to his knees, belted at the waist; a perfect representation of these elite mercenary soldiers who served Irish lords throughout the medieval period. The remaining panels showcase a rich tapestry of medieval imagery, from opposing beasts standing on stylised foliage to interlocking human figures apparently engaged in combat, alongside various interlace patterns and vine leaf designs that were popular decorative motifs of the time.

Whilst no inscription survives to definitively identify who lies beneath, local tradition firmly connects the slab to the McSwyne family, whose ruined castle still stands at Rahan Near on the western side of St. John's Point. The artistic style and iconography place the graveslab firmly in the 16th century, making it a valuable piece of material culture that speaks to the power and influence of the gallowglass families in medieval Donegal. Its careful preservation and display outside the church ensures that this tangible link to Ireland's martial past remains accessible to visitors curious about the region's rich history.