The legend says that Finn built the Causeway due to an ongoing argument with a Scottish giant named Benandonner who could not swim. In an argument one day, while they were shouting at each other over the Sea of Moyle, Finn took a clump of earth and flung it at his enemy. The landfell in the sea and became the Isle of Man, while the huge hole left Ireland became Lough Neagh, the largest lake in the British Isles.
As the arguements continues, Finn decided to build the Causeway in order to make it easy for Benandonner to come and fight him. After so much hard work, Finn was exhausted and he fell asleep.
Oonagh was Finn's wife. When she heard the sound of thunderous footsteps, she knew it was Benandonner come to fight Finn. Oonagh took one look at the gigantic visitor on her doorstep and realized this was a battle Finn could never win. She threw a blanket over her sleeping husband and stuck a bonnet on his head. There was no possible way Finn could defeat Benandonner for he was too small.
"Where's Finn?" roared Benandonner. "Shusha, shusha", whispered Oonagh - "You'll wake the wee one!" Benandonner looked at the snoring Finn. His jaw dropped 10 feet. If the chaild was that hefty, what size would the father be? Benandonner wasn't sticking around to find out. He ran like the wind, destroying part of the Causeway in his wake so Finn could not pursue him.