The version I saw played it as ironic all the way through, up until the final speech, which was very bizarre. All the way through the relationship had been a tongue-in-cheek sort of game between the two of them, ironically sending up the straight interpretation of the play, but then the final scene was a complete turn-around, played seriously rather than ironically, and seemed to make no sense as it wasn't in keeping either with the character of Katherine or with the relationship as they'd been played up til that point.