I attack an Abandoned Hall battlefield with my sole Irelia Fervent. I play En Garde and choose her to get +2 might. Since En Garde chooses her, she gets an additional +1 might. Now that I've played a spell, Abandoned Hall allows me to give a unit an additional +1 might, and I choose Irelia for it. Does this mean that I can give her an additional +1 might for choosing her with Abandoned Hall's ability? If I calculate right, she gets +5 might in total.

To determine Irelia, Fervent's total Might, we must break down the sequence of events step-by-step.

The Sequence

1. Attack: You attack the Abandoned Hall battlefield with Irelia.
2. Play En Garde: You play En Garde (an [ACTION] or [REACTION]) and target Irelia.
   • Trigger: Choosing Irelia with En Garde triggers her ability ("When you choose... me, give me +1 [S]").
   • Chain: The chain consists of [Irelia’s ability (Top)] -> [En Garde (Bottom)].
3. Resolution:
   • Irelia's Ability Resolves: She gains +1 Might.
   • En Garde Resolves: It grants its +1 Might, plus the additional +1 Might for being the only unit there (+2 total).
   • Running Total: Irelia (4 base) + 1 (Ability) + 2 (Spell) = 7 Might.
4. Abandoned Hall Trigger: After En Garde finishes resolving, the Abandoned Hall triggered ability triggers.
   • You place the Abandoned Hall ability on the chain. You choose Irelia as the target to receive +1 Might.
   • Trigger: Choosing Irelia with the Abandoned Hall ability triggers her ability again (+1 [S]).
   • Chain: [Irelia’s ability (Top)] -> [Abandoned Hall ability (Bottom)].
5. Final Resolution:
   • Irelia's Ability Resolves: She gains +1 Might.
   • Abandoned Hall Ability Resolves: She gains +1 Might.
   • Final Total: 7 + 1 + 1 = 9 Might.

Conclusion

Your calculation of +5 Might is incorrect because it does not account for Irelia's base Might or the secondary trigger from the Abandoned Hall ability.

Irelia, Fervent will have a total of 9 Might.

• Base: 4
• From En Garde sequence: +3 (+1 from her ability, +2 from the spell).
• From Abandoned Hall sequence: +2 (+1 from her ability when chosen by the battlefield, +1 from the battlefield itself).

A note on your math: In your prompt, you assumed she gains +5 Might, but that likely ignored her printed base Might of 4. When you "give" a unit Might, it adds to their existing total. Following these steps, she reaches 9 Might total for the turn.

This interpretation is based on the interaction of Triggered Abilities and the LIFO resolution of the chain. Per RiftJudge FAQ #10297, choosing a unit via an ability like Abandoned Hall's triggers Irelia, Fervent's ability.