I have memory of always knowing her as Langinstein, and thats from even before the big Internet boom... WTF?!
EDIT: So I can't link it because you need to be a member there so see it but if you google "card 44 irma panini" you find a card from Panini's sticker collection of the Nick Turtles, this is a card of Irma and it does identify her as Irma Langinstein, this is a Licensed collectible so at least Nick's Irma actually has the surname Langinstein. The site is Laststicker.com BTW and it's a site for people to trade stickers and trading cards.