A cashel is a type of ringfort built from stone rather than earth and timber, the dry-stone walls enclosing a roughly circular area that would once have served as a farmstead or defended homestead during the early medieval period, roughly between the fifth and twelfth centuries. Ireland contains thousands of ringforts of various kinds, but those on island sites carry a particular quality of isolation, built by communities who chose, or were compelled, to place water between themselves and the wider world.

The place-name Illaun derives from the Irish "oileán", meaning island, a name so descriptive it borders on the functional. Clare's Atlantic fringe and its network of lakes and low-lying ground made island settlements both practical and defensible, and cashels in such locations are known from several parts of the west of Ireland. The stone enclosure on Illaun fits into that broader pattern of early medieval life along Ireland's western seaboard, where farming families built in the materials closest to hand and in configurations that offered at least a degree of security against cattle-raiders and rival clans.