The nicely slimmed-down production of On a Clear Day You Can See Forever at Irish Rep is a tidy delight. The show has been revived numerous times since originally opening in the fall of 1965; along the way it has lost several songs (with good reason—they were terrible) and had several interpretations including one by The Vineyard Theater at Vassar College’s Powerhouse Theater that changed the female lead to a gay male. Other versions have added or subtracted characters who weren’t central to what has to be called the “story line.”