What survives runs from the east-northeast around through the south to the west-northwest, a curving wall roughly one and a half metres tall and almost as wide, enclosing a subcircular space about thirty metres across. A later field wall cuts straight across the site from one side to the other, and north of that line no trace of the original structure remains above ground.

A cashel is simply a ringfort built from stone rather than earth and timber, the drystone equivalent of the more common earthen rath, and both types were constructed widely across Ireland during the early medieval period, broadly from around the fifth to the twelfth centuries. They functioned as enclosed farmsteads, the walls marking territory and offering a degree of protection for people and livestock. The Knockakilleen example is built in a manner typical of the Galway limestone landscape: the lower courses are made from large, regularly shaped limestone blocks, the kind of material that required considerable effort to quarry and position. At some point the wall was also thickened on the inside, widened from its original dimensions to around three and a half metres, suggesting either reinforcement or adaptation for a secondary purpose. Rubble from field clearance has been piled against the outer face on the southern side, a reminder that the surrounding land has been actively farmed across the centuries, with the old structure gradually pressed into service as a convenient boundary or a place to deposit gathered stone.