User blog comment:B214/Question and Answers 2/@comment-187.195.197.49-20140430172611/@comment-24098614-20140430201757

The comment about Irina knowing Issei inside and out  means that she has been with him for much longer than any of the girls, and I know that she has dirt on him stuff embarassing things that he doesn't want the girls to know. This is what happens when you know somebody for as long as Irina trust me she has dirt on Issei.