Not a short connecting passage, but a subterranean route stretching roughly 170 metres, linking one ringfort to another across the landscape above. No trace of it is visible at the surface today, which places it in that particular category of archaeological feature that is known almost entirely by reputation.

A souterrain is an underground stone-lined passage or chamber, typically associated with ringforts, those circular enclosed settlements that were the dominant form of rural habitation in early medieval Ireland. They were used variously for storage, refuge, or as escape routes, and vary considerably in scale and complexity. The Euglaune example was recorded in 1937 by a researcher named Broker, who noted the tradition of an underground passage within the ringfort at this location, with the additional detail that it was believed to connect with a second ringfort lying approximately 170 metres to the north. Whether Broker was recording physical evidence or local oral tradition is not entirely clear, but the claim of a tunnel running between two separate enclosures, if accurate, would make this a notably ambitious piece of underground architecture. Inter-fort connections of this kind are not unheard of in the Irish archaeological record, but they are far from common, and the suggestion lends the site an added layer of interest beyond the typical souterrain.