This is the souterrain at Fán, Co. Kerry, a structure whose very oddness rewards attention. Souterrains are underground passages or chambers built from drystone walling and roofed with lintels, found widely across early medieval Ireland, typically associated with ringforts and promontory forts. They were likely used for cold storage, refuge, or both. What distinguishes this one is partly its length and partly its geometry: it does not simply run straight.

For most of its 16.5-metre extent, the passage holds a relatively consistent width and height of around one metre each, tight but navigable. Then, roughly three metres inside the outer face of the rampart, something changes. The souterrain makes an abrupt turn to the south-west, and from that point it narrows dramatically to just half a metre in both width and height. The transition between the two sections is marked by two large orthostats, upright stones that rise the full height of the walls, functioning as a kind of architectural threshold. There is also what the Dingle Peninsula archaeological survey of 1986, compiled by J. Cuppage, describes as an "accidental entrance" just north of that turn, suggesting the passage was breached at some point rather than opened deliberately. The deliberate entrance lies within the fort's own passageway, connecting the underground route to the defended interior above.