The tunnel at Releagh is a rock-cut passage nearly 170 metres long, 5.8 metres wide, and 3.7 metres high, hewn directly through solid rock to carry traffic across one of the more demanding stretches of the Kerry-Cork border country. Road tunnels of any kind are uncommon in rural Ireland, and one cut by hand through bedrock in the early nineteenth century is rarer still.

The tunnel dates from the 1830s, a period of intensive road-building across Munster, much of it funded through public works schemes that employed large numbers of labourers in an era of acute rural poverty. Cutting through the mountain rather than climbing over it would have saved considerable gradient on a route connecting the towns of Kenmare and Glengarriff, both of which were growing in importance as the south-west opened up to trade and, gradually, to tourism. The scale of the tunnel, nearly six metres across, suggests it was designed to accommodate wheeled vehicles rather than merely pedestrian or pack-animal traffic. Three further, shorter tunnels lie to the north-west along the same corridor, indicating that the engineers of the period addressed several rocky obstacles in sequence rather than routing the road around them.