These structures were constructed without mortar, relying on carefully placed drystone walling and corbelled or lintelled roofing, and they have a habit of surviving quietly beneath farmland for over a thousand years, occasionally collapsing underfoot or surfacing during drainage work, more often simply waiting.

An Tuar Glas, a small townland in Mayo, holds one such site on record. Beyond its classification as a souterrain and its location, the available documentation for this particular monument is currently sparse, which places it in a category shared by a surprising number of Irish archaeological sites, known to exist, mapped, assigned a record number, but not yet fully described in any accessible public form. That gap is itself a kind of historical fact. Mayo has a dense concentration of early medieval activity, and souterrains across the province range from modest single chambers to complex multi-passage systems extending many metres underground. Without further detail specific to this site, it is not possible to say which kind this is, how well preserved it remains, or what broader settlement context it may once have belonged to.