From what I can see, the issue is that your mundo fed her early kills and she snowballed. She's definitely strong atm, but I think the issue was that she got out of control because everyone kept trying to take her down piecemeal instead of grouping and shutting her down. Please correct me if I'm wrong, but for at least the first half of the game it appears that she was almost never confronted by more than 1 or 2 people at once, which allowed her to snowball off of Mundo's unlucky early death.
The first time you confronted her, it was just you (an 0-1 Brand with only a Needlessly large rod) and Mundo (who was 0-3 and pretty much just had armor and a ruby crystal, which is a bad build, he should have rushed warden's mail instead of flat armor) facing a 3-0 Irelia who was just shy of a full Rageblade on top of having Berserker Greaves and an extra dagger. After that it seems she just took over since she was now 5-0, just hit a huge powerspike by finishing Guinsoo's, and then proceed to take top tower and kill mundo again, roam to kill you again, and then spiraled out of control taking several more kills, towers, and other objectives throughout the match.
The issue here isn't Irelia, the issue is that Mundo got outplayed and then everyone else suffered the consequences.