Saint Patrick's Cave is a underground cave beneath Brendan Finn's lighthouse cove. A pivotal location during the Dangerous Habits storyline, this is the scene where John first meets the First of the Fallen, before subsequently tricking him.
The cave is located in Killiney, Norther Ireland. The Irish magician, Brendan Finn, discovered the cave after trying to expand the size of his cellars underground, and after researching it in the Trinity library, he finds out that it was actually a holy place blessed by St. Patrick himself during his travels in Ireland.
The cave seemed to be unnaturally enlarged through human means, even containing tiled floors and sitting benches. The cave contained a pool of holy water in the center, and like other holy places, was a secure location from the creatures of the underworld. Due to Brendan selling his soul to the First some time ago for the largest liquor collection in the world, the mage feared that he would be too weak to fight back when First of the Fallen finally comes for him. The discovery of the cave gave Brendan some hope to save his soul from eternal damnation, hoping to use the cave as a hiding place from Satan.
Another glimpse of hope came when Brendan's longtime friend and fellow magician, John Constantine, came seeking help. Brendan shows John the cave he discovered, and teaches the Brit how to turn holy water into Guiness. After some time reveling, Brendan quietly passed away as John was about to leave. John was, however, interrupted by the First of the Fallen, who managed to track down Brendan, laughing at the poor man's attempt to save himself, although the Devil noted how the holy place made his job difficult.
Five minutes before the allotted time of taking Brendan's soul; John, not wanting to see his friend be sent to Hell, devices a scheme to save him. He invites Satan for a drink, while also continuing their insult on the poor Irish mage. Their drink however, was actually holy water turned into wine, as such the First was immediately poisoned after drinking it. John then mutilates and kicks the First into the pool of holy water, sending him back to Hell in a very weakened state.
Happy of saving his friend's soul, it even gave John hope to also fight back and save his own soul.
Like other holy places, the cave contained powerful supernatural protection from even the Devil himself. It is extremely difficult for demons to track the place down, and its pool of holy water can be used as a weapon.