Her level three Ultimate got nerfed by 25 damage kids. 25 damage multiplied by 4 blades is 100 damage less then what it used to be overall.
At first this seems like a nerf, but in reality it's around the same amount of damage when her ult was considered "op" by the majority of players. This is because they made her ulti scale off both AP and AD at the same time meaning that her ult gets a bonus 15 damage from the 30 ap you obtain from Trinity Force putting her level 3 ult back up to 175 base damage, which is only a small decrease from her zenith.
Also, there is another post out there about Irelia stating the truth about her and the stupidity of these "my ult doesn't do any damage now" threads. Irelia's ult was great and OP because it gave you a ridiculous amount of lane sustain and heals in team fights, not because it dealt copious amounts damage with little investment into AD/AP.
These nerfs are really unnoticable in reality and are more of an excuse for a player who is doing badly, or a way for the regular LoL player to spew >inster champion here< is up because they got nerfed, or >insert champion here< is op because they got buffed.
Atm, if you want your ult to do more damage. Build hextech gunblade or hexdrinker. If you want more damage with your Hiten Style and auto attacks, build wit's end. RIOT is trying to push Irelia away from a "tank assassin" and pick and choose which attributes she wants to excel in instead of being good at them all.
edit: also, a lot of people probably think their ult does no damage when in reality they missed two of their blade skill shots. Gotta hit 'em all.