The National Trust for Scotland owns the cave as part of a national nature reserve. It became known as Fingal’s Cave after the eponymous hero of an epic poem by 18th-century Scots poet-historian James Macpherson.
According to the legend, the two places are the opposite ends of an ancient bridge built by the benevolent Irish giant Fionn mac Cumhaill. While building the pathway towards Scotland, Fionn gets informed that his enemy Scottish giant Benandonner is coming to fight him. Fionn cannot withstand Benandonner’s strength, so he asks his wife Oona to help him. She disguises Fionn, dressing him as a baby and hiding him in a cradle. Then she bakes some cakes, hiding some iron in some of them, and waits for the giant’s arrival.
When Benandonner arrives, not finding Fionn, he waits for him in his house. At the same time, he tries to intimidate Oona by showing her his great power. At this point, Fionn’s clever wife offers Benandonner some iron cakes, but as he bites into cakes, the iron he chips his teeth. Oona ridicules him for being weak, saying that her husband eats those cakes without trouble, and feeds one (without iron) to the camouflaged Fionn. Benandonner, having seen the baby’s strength, is scared to meet his father and runs back to Scotland, smashing the causeway behind him so Fionn couldn’t follow.
Fingal’s Cave is formed entirely from hexagonally jointed basalt columns within a Paleocene lava flow and is similar in structure to both the Giant’s Causeway in Northern Ireland and Ulva.
In these locations, cooling on the upper and lower surfaces of the solidified lava resulted in contraction and fracturing, starting in a blocky tetragonal pattern and transitioning to a regular hexagonal fracture pattern with fractures perpendicular to the cooling surfaces. As the cooling continued these cracks gradually extended toward the center of the flow, forming the long hexagonal columns we see in the wave-eroded cross-section today. Similar hexagonal fracture patterns are found in desiccation cracks in the mud where contraction is due to loss of water instead of cooling.
The cave has a large arched entrance and is filled with the sea. Several sightseeing cruises organized from April to September by local companies pass the entrance to the cave. In calm conditions, one can land at the island’s landing place (as some of these cruises permit) and walk the short distance to the cave, where a row of fractured columns forms a walkway just above high-water level permitting exploration on foot. From the inside, the entrance seems to frame the island of Iona across the water.
According to Wikipedia