If it's my turn and I play Stupefy and Hextech Ray targeting Irelia, Fervent, how will damage and might be distributed on the chain, and what units with what might will I need after those spells resolve?

No — both spells together still leave Irelia, Fervent alive, at 5 Might with 3 damage marked.

She's 4 Might base, and "When you choose or ready me, give me +1 [M] this turn" (a Targeting Effect, Rule 383.4.b) fires on each spell that chooses her — the trigger goes on the chain above the spell and resolves first. Net from the pair: +1 (Stupefy) −1 (Stupefy) +1 (Hextech Ray) = 5 Might, and all three effects sit in the same arithmetic layer, so the order they resolve in doesn't change the total. If both are on the chain at once (Hextech Ray first, since Stupefy is the one with Reaction — you can't play an Action while a chain is open), it resolves top-down as: Irelia trigger → 5, Stupefy → 4 and you draw 1, Irelia trigger → 5, Hextech Ray → 3 damage to her. Also remember Deflect: you pay [A] on top of each spell's cost, once per spell that chooses her.

So Hextech Ray's 3 is never lethal here — lethal is damage ≥ Might (Rule 142.4.b), so she'd have to be at 3 Might or less when it resolves, and each time you choose her she grows. After resolution she keeps 3 damage marked, so you only need 2 more damage from anything to finish her (marked damage counts toward lethal in combat assignment, Rule 467.2.c.4) — a 2-Might attacker is enough. If you're trying to set up a kill with Stupefy, note it can only ever offset one of her +1s, never undercut her.