The Giants Causeway was included on the World Heritage site list in November 1986. It is on the list as both a cultural and natural site, one of only 25 in the world to achieve this status. This is because the Causeway meets two of UNESCO’s criteria for this:

(1) It is a prime example of earth’s evolutionary history during the tertiary epoch.

(2) It contains rare and superlative natural phenomena

The site also has outstanding cultural value in that it contains the wreck of the Girona

The Giant's Causeway lies at the foot of the basalt cliffs along the sea coast on the edge of the Antrim plateau in Northern Ireland. It is made up of some 40,000 massive black basalt columns sticking out of the sea. The dramatic sight has inspired legends of giants striding over the sea to Scotland. Geological studies of these formations over the last 300 years have greatly contributed to the development of the earth sciences, and show that this striking landscape was caused by volcanic activity during the Tertiary, some 50–60 million years ago.

Source World Heritage Site - see useful links page