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According to answers on this question about how faerie fire and mirror image interact, the faerie fire spell only affects the creature, and the mirror image spell can change the target of an attack.
But if the attacker's modifiers can change between the intended target and the target attacked, when is the attack actually rolled?
(Does the attack roll trigger "targeted with an attack", or is mirror image resolved first as part of resolving modifiers for the attack?)
The basic rules describe the process of making a basic attack thus:
Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
Resolve the attack. You make the attack roll.
Faerie Fire states:
Any attack roll against an affected creature or object has advantage if the attacker can see it.
Mirror Image states:
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
Putting it all together:
Step 1: The attacker declares the Faerie Fire'd creature as the target. At this point, the Mirror Image kicks in. The creature affected by Mirror Image has been targeted. However, Faerie Fire's granting of advantage requires an attack roll. No such roll has yet been made.
The Mirror Image affect kicks in and (potentially) redirects the attack to an image instead of the original target.
Step 2: determine bonuses and advantage/disadvantage. If the target has been redirected to an image which, though outlined by the Faerie Fire (because they mirror the appearance of the original target which has been outlined), is not affected by it.
Step 3: The attack roll is made without advantage. The attack roll is made but it is no longer targeting a creature affected by Faerie Fire.
As a result, if an attack is made against a target simultaneously affected by Faerie Fire and Mirror Image, the roll to redirect the attack preempts the attack roll and therefore prevents Faerie Fire's benefits from applying to the attack roll against the mirror image.
However, one could subscribe to the logic of this answer on the related question. The logic of that answer, which leans towards a logical narrative explanation and away from a strict legal reading, states that the images are also affected by Faerie Fire by virtue of the fact that outlining light effect is what makes a target easier to hit.
If you use this interpretation, then the question is entirely avoidable because all possible targets are legally affected by Faerie Fire, not just visually.
From the PHB page 194 under "Making an Attack" (Emphasis mine):
- Choose a target. Pick a target within your attack’s range: a creature, an object, or a location.
- Determine modifiers. The GM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
- Resolve the Attack. You make the attack roll...
Thus first you choose a target, and as shown in this Q/A you cannot target the duplicates themselves, thus you would be targeting the creature.
The description of the mirror image spell states (emphasis mine):
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates...
Thus the d20 roll to see whether an attack is redirected occurs before the attack roll is made, and thus before any modifiers, such as advantage or disadvantage are applied. After your target has been chosen by mirror image's d20 roll the modifiers are determined.
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.
A duplicate's AC equals 10 + your Dexterity modifier. If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The spell ends when all three duplicates are destroyed.
It seems clear from the spell description that the target rolls to determine whether they or one of the duplicates are being attacked, then the attacker rolls to attack the AC of whatever they are attacking. I don't see how this can be interpreted any other way.
It doesn't really matter which d20 physically gets rolled first, but success of the attack roll cannot be calculated without knowing the AC of the target, and the AC of the target cannot be known until it has been determined which target is actually being attacked.
