I asked David, not sure if he'll respond. He mostly never does.
EDIT: Okay wasn't expecting that. David replied to me via Facebook. He said I could quote his responses as well. I asked him what Irma's surname was and his response was:
"I don't think she ever had one. It probably got mentioned in some obscure episode somewhere (maybe not even one of mine), but I have no memory of it."
David wrote the first episode she was in so I wondered if he created her character. He didn't answer that but he did clarify whether Langinstein is her surname:
I am almost 100% certain she never had THAT particular name.
Then said:
If her last name was ever mentioned in an episode I'm sure some fan would have made note of it. My guess is we never gave her a last name because we never had to. I usually don't both with a last name unless it has to be used in dialogue. If I had created 007 he would have been just "James!"
He also pointed to where he got the name Irma from:
http://www.imdb.com/title/tt0041673/
Hope that's helpful guys.