Irelia gains a massive power spike with Blade of the Ruined King (BotRK), often turning around losing lane states.
She's particularly strong against mage champions in mid lane; mastering Q stack management and utilizing W's damage reduction are crucial.
Irelia is weak against certain bruisers like Warwick and Darius, so it's often better to pick her as a counter-pick rather than a first pick.
Stats
Q
W
E
R