According to legend, the columns are the remains of a causeway built by a giant.
The story goes that the Irish giant Fionn mac Cumhaill (Finn MacCool), from the Fenian Cycle of Gaelic mythology, was challenged to a fight by the Scottish giant Benandonner.
Fionn accepted the challenge and built the causeway across the North Channel so that the two giants could meet.
In one version of the story, Fionn defeats Benandonner.
In another, Fionn hides from Benandonner when he realises that his foe is much bigger than he is.
Fionn's wife, Oonagh, disguises Fionn as a baby and tucks him in a cradle.
When Benandonner sees the size of the 'baby', he reckons that its father, Fionn, must be a giant among giants.
He flees back to Scotland in fright, destroying the causeway behind him so that Fionn would be unable to chase him down.
Across the sea, there are identical basalt columns (a part of the same ancient lava flow) at Fingal's Cave on the Scottish isle of Staffa, and it is possible that the story was influenced by this.

In overall Irish mythology, Fionn mac Cumhaill is not a giant but a hero with supernatural abilities, contrary to what this particular legend may suggest.
In Fairy and Folk Tales of the Irish Peasantry (1888), it is noted that, over time, "the pagan gods of Ireland [...] grew smaller and smaller in the popular imagination, until they turned into the fairies; the pagan heroes grew bigger and bigger, until they turned into the giants".
There are no surviving pre-Christian stories about the Giant's Causeway, but it may have originally been associated with the Fomorians (Fomhóraigh); the Irish name Clochán na bhFomhóraigh or Clochán na bhFomhórach means "stepping stones of the Fomhóraigh".
The Fomhóraigh are a race of supernatural beings in Irish mythology who were sometimes described as giants and who may have originally been part of a pre-Christian pantheon.
According to legend, the columns are the remains of a causeway built by a giant. The story goes that the Irish giant Fionn mac Cumhaill (Finn MacCool), from the Fenian Cycle of Gaelic mythology, was challenged to a fight by the Scottish giant Benandonner. Fionn accepted the challenge and built the causeway across the North Channel so that the two giants could meet. In one version of the story, Fionn defeats Benandonner. In another, Fionn hides from Benandonner when he realises that his foe is much bigger than he is. Fionn's wife, Oonagh, disguises Fionn as a baby and tucks him in a cradle. When Benandonner sees the size of the 'baby', he reckons that its father, Fionn, must be a giant among giants. He flees back to Scotland in fright, destroying the causeway behind him so that Fionn would be unable to chase him down. Across the sea, there are identical basalt columns (a part of the same ancient lava flow) at Fingal's Cave on the Scottish isle of Staffa, and it is possible that the story was influenced by this.
In overall Irish mythology, Fionn mac Cumhaill is not a giant but a hero with supernatural abilities, contrary to what this particular legend may suggest. In Fairy and Folk Tales of the Irish Peasantry (1888), it is noted that, over time, "the pagan gods of Ireland [...] grew smaller and smaller in the popular imagination, until they turned into the fairies; the pagan heroes grew bigger and bigger, until they turned into the giants". There are no surviving pre-Christian stories about the Giant's Causeway, but it may have originally been associated with the Fomorians (Fomhóraigh); the Irish name Clochán na bhFomhóraigh or Clochán na bhFomhórach means "stepping stones of the Fomhóraigh". The Fomhóraigh are a race of supernatural beings in Irish mythology who were sometimes described as giants and who may have originally been part of a pre-Christian pantheon.
Giant's Causeway