Underware, one of my favorite type foundries, released to the typography world a new mark, to express irony in writing.

Read the entire article and see the examples of use for the new mark on Underware's website.
So far, just a few typefaces listed there let you use the new irony mark, but the list could grow if more foundries will integrate it in their font families. A big boost could come from Adobe, Berthold, Linotype, FontFont and others if they will decide to support this new punctuation mark.
Simple and effective, I think it could work very well.