It looks like these 14 countries were the only ones in the surveyed, so its not that Ireland didn't make it on the list (map,
Are You As Green As India & China? : EcoWorldly)
...maybe National Geographic thought that since the main British island was covered, the outlying ones would be about the same. But if it’s any compensation, you could always check your personal “greendex” using the questionnaire from the survey:
Greendex: Survey of Sustainable Consumption - National Geographic