I use Disciple on my Irelia Irelia, Fervent, which my opponent reacts with Defy. Do I still get the +1 Might for choosing my Irelia with Disciple?

Yes, you do still get the +1 Might for Irelia, Fervent.

According to the rules of Riftbound and the associated community FAQs (e.g., FAQ #8823, FAQ #5762), the sequence of events works as follows:

1. Playing the Spell: You play Discipline targeting your Irelia, Fervent.
2. Triggering Ability: Because you chose Irelia, her ability ("When you choose or ready me, give me +1 Might this turn") triggers immediately upon the spell being finalized on the chain. Her ability is placed on the chain on top of your Discipline spell.
3. Opponent's Reaction: Your opponent plays Defy in response, targeting your Discipline spell. Defy is placed on the chain on top of Irelia's triggered ability.
4. Chain Resolution (LIFO): The chain resolves in Last-In, First-Out order:
   • Defy resolves first, countering your Discipline spell.
   • Irelia, Fervent’s triggered ability resolves next. Since her ability is already on the chain, it resolves successfully, granting her +1 Might.

Key Clarifications:

• Trigger Independence: Once Irelia’s triggered ability is placed on the chain, it exists independently of the spell that triggered it. Even if the original spell (Discipline) is later countered or leaves the chain, the triggered ability remains and resolves normally.
• Defy Limitations: As noted in FAQ #1137, Defy can only counter spells. It cannot counter triggered abilities like Irelia's. Even if your opponent wanted to prevent the Might bonus, they cannot target or counter the trigger with Defy.

This is my interpretation based on the chain rules and the cited FAQs. No official rulebook text exists for this specific card combo interaction.