if I choose to ready irelia fervent using irelia legend, how much might increase would that give?

+1 Might — only from the "ready" half of her ability, not from being chosen.

Irelia, Fervent reads "When you choose or ready me, give me +1 [M] this turn." Blade Dancer (Irelia's Legend) readies a friendly unit but does not choose or target that unit, so her "chosen" trigger doesn't fire — only the "readied" trigger does. Net result: +1 Might (FAQ 8590).