Looks like something straight out of Tolkien.
The Labby Rock’s capstone, most likely a glacial erratic, weighs approximately 65 tons. The structure was built between 3,000 to 2,500 BC at the bottom of a small valley in the townland of Carrickglass.
Folklore: Diarmait and Grainne are believed to have slept here when fleeing from the Giant Fionn MacCumhaill, to whom Grainne was promised. Legend also says Nuada of the Silver Arm, King of the Tuatha De Danann is buried here, Nuada was killed in the Battle of Moytirra by Balor of the Evil Eye, King of the Formorians.