How much might does irelia gain from defiant dance + being ready with the legend effect?

+4 Might total (assuming the -2 half goes to a different unit, so only one "chosen" trigger fires).

Defiant Dance gives her +2. Choosing her with the spell triggers her ability for +1 (chosen). Blade Dancer then readies her (choosing her with the Legend does not itself count as "choosing" — FAQ 8590), and the ready half of her ability triggers for another +1 (readied). So: 4 base → 8 Might for the turn.

Edge case: if the -2 half is also aimed at a friendly unit, that's an extra "choose a friendly unit" that could let Blade Dancer trigger again, but she's already ready, so no further ready trigger — she'd stay at +4 from this line.