At the risk of getting into a long and pointless circular argument, I'll cut to the chase: Why are people so interested in the Celtic languages?
Possible reasons include:
Any of the features that make them interesting, initial consonant mutations, that sort of thing. Old Irish is infamously complex for those that like to say they understand complicated languages.
A long history, plenty of literature and mythology as well as contemporary literature and media. Huge oral traditions, and it ties in with the GAA, Irish dancing, traditional music and singing.
I've met a lot of Americans who are very into their Irish heritage. Most of them like to mispronounce a cúpla focal, a few are bound to be linguists who actually learn the language well.
The same argument can be made for most languages, and also as a serious question, why wouldn't they be interested in learning Celtic languages? People are interested in minority languages, Celtic ones combine enough living natives (as well as a huge number of fluent learners in Ireland anyway) to make it a very worthwhile and fulfilling language to learn.
I do often find it amusing however, that there are thousands of Irish students who study Irish for 14 years or so, and leave school unable to have even a basic conversation. It really illustrates the saying, "You can lead a horse to water, but you cannot make him drink".