According to legend, the Giant’s Causeway was built by the giant Fionn mac Cumhaill (Finn McCool), the leader of the Fianna war band.

An excerpt from the article 39 facts about Ireland

He wanted to get to Scotland safely and defeat his counterpart there. When he passed and saw how big his opponent was, he fled in panic and, with the help of his wife, disguised himself as a child. When the Scottish giant came to Ireland and saw the baby, he was terrified by the sight and wondered how big were the adults. Fleeing to Scotland, he chopped down a Causeway so that the Irishman could not follow him back.