This is a spell synergy question.
Casting order is mirror image spell first so the illusory duplicates AC are set, and then casting shield of faith spell, creating a barrier "around" the creature mirror image was cast on and if I am right around the occupied space of said creature, so this would include the illusory duplicates.
Shield of Faith states:
A shimmering field appears and surrounds a creature of your choice within range, granting it a +2 bonus to AC for the duration.
Mirror Image states:
Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.
Would the "shield of faith" spell provide an AC boost to the illusory duplicates created via the "mirror image" spell given that they are all occupying the same space and moving in sync (mimicing your actions).
If this is indeed how these 2 spells sync, the illusory duplicates would instead have 15 AC each instead of 13 AC (base creature has a dex bonus of +3) while also boosting the original creature's AC by +2.