I play impperial decree, my opponent has open zhonya and scuttle irelia on a bf With noxus hopeful I deal 1 to scuttle and 3 to the 4 might irelia. Can my opponent save irelia with zhonya or is too late because scuttle dies before the decree trigger is on the chain ?

Yes — the opponent can still save Irelia. The Scuttle dying first doesn't close the window.

When the damage is dealt, Irelia taking 3 (non-lethal on a 4-might body) puts Imperial Decree's "kill it" trigger on the chain right away, and the opponent gets priority while that chain item is pending to reveal the hidden Zhonya's (it has Reaction while hidden). The Zhonya's resolves first, sits on the board, then replaces Irelia's death when the Decree trigger resolves — she's healed, exhausted, recalled to base, and the Zhonya's is killed instead. Because 3 damage isn't lethal, there's no damage death to burn the Zhonya's on, so it cleanly intercepts the Decree kill (FAQ #8420; the Scuttle's Deathknell trigger would have given them that window too, but the Decree trigger alone is enough — FAQ #5681).

One caveat: if the Zhonya's is already face-up rather than hidden, it's a mandatory replacement effect and it gets used up on the first friendly unit that would die — the 0-might Scuttle in the post-damage cleanup — leaving Irelia unprotected when the Decree trigger resolves. If it's a facedown hidden card, they choose the timing and Irelia lives.