Data.CIByteString is a module containing CIByteString, a wrapper for ByteString which provides case-insensitive (ASCII-wise) Ord and Eq instances.

CIByteString also has an IsString instance, so if you use the "OverloadedStrings" LANGUAGE pragma you can write case-insensitive string literals, e.g.:

 > let a = "Foo" in
   putStrLn $ (show $ unCI a) ++ "==\"FoO\" is " ++ show (a == "FoO")
 "Foo"=="FoO" is True