The souterrain inside one of the ringforts at Cush, in County Limerick, does all of that, but then adds something more considered. Concealed within its fabric is a ventilation system deliberately designed to disguise its own existence from anyone threatening the fort from outside.

A souterrain is an underground stone-built passage or chamber, typically associated with Early Medieval settlement in Ireland, often found within the earthen banks of a ringfort. The Cush example sits in the north-east quadrant of one such fort and was excavated by Ó Ríordáin between 1934 and 1935, with the findings published in 1940. The structure breaks into three connected sections: a roofed stone passage roughly 5.4 metres long, an unroofed walled section of similar length running at right angles to it, and a further approach of about 4.87 metres indicated not by stonework but by post-holes cut into the underlying red sandstone. Those five post-holes, arranged in two parallel rows, are the ghost of a timber entrance structure that once completed the whole design. The interior of the roofed section was cramped even by the standards of these passages; the average height between floor and roof was about 0.76 metres, narrowing to roughly 0.3 metres where two large slabs dipped at their junction near the western end. Running north-west from that same western end, a drain-like arrangement of stones extended nearly 5.8 metres to the fort's rampart. Ó Ríordáin identified this as a ventilation shaft, and noted the particular care taken in its routing: by terminating at the rampart rather than opening obviously on the interior, it kept the air supply hidden from any attacker whose quarry had taken refuge underground.

Cush is a townland in the Kilmallock area of County Limerick, and the landscape there holds a cluster of ringforts that Ó Ríordáin investigated across several seasons in the 1930s. The ventilation shaft at this particular fort, recorded as LI048-034027-, was actually the first of several such features to be uncovered across the Cush complex, with clearer examples later found at other forts on the same site. The distinction in the sediment recovered during excavation gives a useful picture of how the structure was used and abandoned: the unroofed section filled with dark, organic-rich material consistent with accumulation during the period the fort was occupied, while the roofed tunnel silted up gradually with clean material washed in through gaps in the stonework long after the place had been deserted. The site is recorded in the Archaeological Survey of Ireland, and Ó Ríordáin's published plans remain the primary reference for understanding the layout.