The two things seem to contradict each other, saltwater and fresh, cave and well, but here they coexist, and the spring can only be reached when the tide withdraws far enough to allow entry.

The cave sits below the edge of Portraine Demesne, on the coast of the Fingal peninsula. A natural spring well, recorded by Healy in 1975, lies deep within the cave, tucked beneath the old demesne wall where land meets sea. Spring wells of this kind, where freshwater rises through rock close to or even within the intertidal zone, are not unknown along the Irish coast, but they are unusual enough to register. The precise geology that allows freshwater to emerge here, resisting the encroachment of seawater, is a function of the underlying aquifer and the pressure of the water table, but the effect from the inside of a sea cave is something harder to explain away so tidily.

Access depends entirely on the tide. The cave is only reachable at low water, which means planning around tide tables before any visit. Portraine itself sits on a small peninsula between the Rogerstown and Broadmeadow estuaries, north of Donabate, and the coastline in this area is low-lying and can be slippery where rock and weed meet. Visitors should approach cautiously and allow enough time to retreat before the tide returns. The well itself lies deep inside the cave, so a torch is advisable. There is no formal access infrastructure here, and the site sits within the boundary of the old demesne rather than any designated public amenity.