Can Iron Revenant pay for accelerate?

Yes — Iron Revenant's [A] can pay the Power portion of an Accelerate cost, and its Reaction tag means you can activate it at exactly the moment you need it.

Iron Revenant reads "[Reaction][>] Banish a unit or gear from your trash, [T]: [Add] [A]. Spend this Power only to play units or gear." Accelerate is an optional additional cost paid as you play a unit (Rule 805.1.b), so it's part of playing a unit — squarely inside that spending restriction. And because the Power is [A] (any domain), it satisfies the requirement that Accelerate's Power match one of the unit's domains (Rule 805.1.b.1). You still have to actually declare Accelerate during the Choose step; if you do, the unit enters ready.

Two things to keep in mind: activating it costs a banished unit or gear from your trash plus exhausting the Legend, and since it's a Reaction Add ability it resolves immediately mid-payment (Rule 430.3) — your opponent can't respond to the resource generation.