Not can't. You won't be able to target Iroas with Ray of dissolution until it resolves and has hit the battlefield at which point it will be indestructible.
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Every normal man must be tempted, at times, to spit on his hands, hoist the black flag and start slitting throats.
- H.L Mencken
I Became insane with long Intervals of horrible Sanity
All Religion, my friend is simply evolved out of fraud, fear, greed, imagination and poetry.
- Edgar Allan Poe
Nothing can "destroy" a spell, and Ray of Dissolution doesn't even try. Nor can anything destroy a permanent with indestructible, which is what Iroas, God of Victory will be once it resolves.
109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.
702.12b A permanent with indestructible can’t be destroyed. Such permanents aren’t destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g).
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Why bother with mere rulings when so many answers can be found in the Rules?
Ray of Dissolution works on enchantments that are on the battlefield. You cant cast Ray on an enchantment when said enchantment is cast, as it is not yet resolved, and on the battlefield - still on the stack. That's without touching the fact that Iroas is indestructible.
Further, casting Ray of Dissolution "as an instant" isn't an option; the card type is always instant, no matter when you cast it. You can always cast it when you have priority.
- H.L Mencken
I Became insane with long Intervals of horrible Sanity
All Religion, my friend is simply evolved out of fraud, fear, greed, imagination and poetry.
- Edgar Allan Poe
The Crafters' Rules Guru
Ray of Dissolution works on enchantments that are on the battlefield. You cant cast Ray on an enchantment when said enchantment is cast, as it is not yet resolved, and on the battlefield - still on the stack. That's without touching the fact that Iroas is indestructible.
When in doubt, call a judge.
Objectivist here. Hit me up to talk philosophy.