If my Irelia is at 11 might and then gets Challenged for 9 might and then a combat starts while it's at 2 HP, does the Irelia do 2 damage to the enemy or 11?

Irelia will deal 11 damage.

In Riftbound, a unit's "HP" (the amount of damage marked on it) does not reduce its Might or its ability to deal damage in combat. According to Rule 460.2, units resolve combat damage using their current Might.

As long as Irelia has not been killed (which only happens during the Combat Cleanup step if the damage marked on her is equal to or greater than her Might), she remains on the battlefield and contributes her full current Might to the combat damage assignment.