It is a cashel, the term used for a ringfort built from stone rather than earthen banks, and this one measures roughly twenty-five metres across, its enclosing wall still standing around half a metre tall in places. Where the grass thins, the original wall facing remains visible, the stones set to a width of nearly two metres, which gives a sense of how solid the original construction would have been.

Ringforts of this kind are among the most common archaeological monument types in Ireland, with estimates running to tens of thousands across the island. Most date to the early medieval period, broadly the fifth to twelfth centuries, and they functioned primarily as enclosed farmsteads, protecting a household and its livestock rather than serving any military purpose in the modern sense. The cashel at Tullyglass follows the standard form closely. There is a gap to the east, the typical placement for an entrance, which would have caught the morning light and faced away from the prevailing Atlantic weather. What makes this particular gap worth a second look is the recumbent slab on its southern side, a flat stone roughly 1.2 metres long and a quarter of a metre thick, which may be a remnant of the original threshold or jamb arrangement. Whether it fell or was placed there is not recorded, but its presence adds a small degree of structural detail to what is otherwise a quietly eroded monument.