Irish stepdancing is an extremely difficult but beautiful form of folk dancing, which has distant relatives in Appalachian clogging as well as the more common tap-dancing. The Irish Dance "scene", though, is far from folky. Rather, it has been fostered through the spirit of competition and performance, with young dancers competing at competitions called feiseanna (pronounced fesh-ay-onna, the singular form is feis, pronounced fesh). Feiseanna are easily as competitive as your average gymnastics meet, and the same level of both physical and artistic skill is required of the dancers.
There are academies of Irish Dance all over the country, offering primarily classes for younger students. However, many of them also offer adult classes, which are not competition based, but simply in the spirit of learning and connecting to ones cultural roots. FeisWorx has provided a nice list of Irish Dance Schools throughout the US and Canada.