A cashel is a ringfort built from dry-stone walling rather than earthen banks, a distinction that tends to preserve them well in the rocky west of Ireland, where loose stone was always more plentiful than deep soil. These circular enclosures were the farmsteads of early medieval Ireland, typically dating from somewhere between the fifth and twelfth centuries, and they appear in their thousands across the country, though many have been levelled by agriculture or simply absorbed back into the ground.

The cashel at Shanwar belongs to a class of monument that was once so ordinary it was barely remarked upon, the everyday domestic architecture of a rural society organised around cattle, kinship, and the rhythms of a working farm. The stone wall of a cashel would have enclosed a family's living quarters and perhaps their animals, offering protection less from warfare than from wolves and opportunistic raiding. Mayo has a particular density of these sites, partly because the thin soils and exposed terrain made stone construction practical and partly because large portions of the county escaped the intensive agricultural clearances that destroyed so many comparable monuments elsewhere.