Hi Laura,
Slightly it's mystifying as to who they could be. Slightly too young to be Horslips, and slightly too old to be the Frames (or even Lir). But if their from the Boston or New York area my guess would be the New York Celtic Rock band 'Fathom' -though even they are still probably too young to have been your mystery band . Some of their songs are as awesome as Horslips themselves (with an equaly questionable dress sence at times too!). My guess is that if it's not Fathom then they will probably know the real mystery band personaly. You can find / contact them via
http://www.myspace.com/celticrock or
http://www.fathomband.com/