I am not an Irelia player, but I have her. You probably lost against her or at least got completely destroyed in lane.
Tenacity: Treat her like you would treat a TF. Their stun is huge, and undodgeable, but predictable nonetheless. In Irelia's case, it is when you have more HP than her, meaning you get stronger the more harass you take. This means champions that thrive on low HP make her life harder than champions that need to deal damage before-hand. Or you can always just build tenacity.
Her own tenacity: Knock-ups and pulls. This shouldn't be hard considering no champion in the future decades will come out without a knockup from now on.
Her true damage: HP. Not much else to say here. AS slows help too and healing reduction.
Her maxing her Q instead: Armor, plus forcing her to use it on you to make her oom faster.
Her snowball potential: Play safe during laning phase and try to get to mid game without dying often. If you can, she looks pathetic, kind of like Volibear without any kills.
Outsustain: Bring a manaless champion to the table that can force Irelia to back before you. Her waveclear is terrible, so getting a farm advantage might be quite easy.
Ult: Not sure what is the problem here... Most balanced part in Irelia's kit, if you ask me. Despite being hard, it can still be dodged, and you should ignite her if you are close to death in an even fight.
Her stun: This is kind of retarded, but still, I have already given you a few tips, and if she wants that early 2 second stun, she needs to max it first, letting you duel her quite easily.
Now... get better from your loss, and quit complaining about a champion that defeated you. She is annoying, but there is a lot worse.
