Legend has it that the Irish warrior Fionn mac Cumhaill (Finn McCool) built the causeway to walk to Scotland to fight his Scottish counterpart Benandonner. One version of the legend tells that Fionn fell asleep before he got to Scotland. When he did not arrive, the much larger Benandonner crossed the bridge looking for him. To protect Fionn, his wife Oonagh laid a blanket over him so he could pretend that he was actually their baby son. In a variation, Fionn fled after seeing Benandonner's great bulk, and asked his wife to disguise him as the baby. In both versions, when Benandonner saw the size of the 'infant', he assumed the alleged father, Fionn, must be gigantic indeed. Therefore, Benandonner fled home in terror, ripping up the Causeway in case he was followed by Fionn.

Another variation is that Oonagh painted a rock shaped like a steak and gave it to Benandonner, whilst giving the baby (Fionn) a normal steak. When Benandonner saw that the baby was able to eat it so easily, he ran away, tearing up the causeway.

Another version of the legend was that Fionn had spent many days and nights trying to create a bridge to Scotland because he was challenged by another giant. A fellow boatsman told him that the opponent was much larger than he. Fionn told his wife and she came up with an ingenious plan to dress Fionn like a baby. They spent many nights creating a costume and bed. When the opponent came to Fionn's house; Fionn's wife told him that Fionn was out woodcutting and the opponent would have to wait for him to return. Then Fionn's wife showed him her baby and when the opponent saw him he was terrified at the thought of how huge Fionn would be. He ran back to Scotland and threw random stones from the causeway into the waters below.